Nonvolatile semiconductor memory device having electrically and collectively erasable characteristics

ABSTRACT

A semiconductor memory device has 2 n  word lines, a plurality of bit lines, a plurality of nonvolatile memory cells disposed at each intersection of the word lines and the bit lines, a write circuit for writing data to a memory cell located at an intersection of selected ones of the word lines and the bit lines, and a sense amplifier for reading data out of the memory cells. Further, the semiconductor memory device comprises a first unit for simultaneously selecting a block of 2 m  (n&gt;m) word lines among the 2 n  word lines, and a second unit for not selecting a block of 2 k  (m&gt;k) word lines among the 2 m  word lines. The second unit does not select the block of 2 k  word lines, and selects a block of 2 k  word lines prepared outside the 2 n  word lines when any one of the 2 k  word lines among the 2 m  word lines is defective. Consequently, redundant word lines are effectively employed, write and verify operations are stable, and thereby the yield and performance of the semiconductor memory device are improved.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor memory device,and more particularly, to a flash memory, i.e., a nonvolatilesemiconductor memory device having electrically and collectivelyerasable characteristics.

[0003] 2. Description of the Related Art

[0004] Recently, in accordance with popularization of computers, wordprocessors, and the like, a plurality of semiconductor memory devices,typically non-volatile semiconductor memory devices, e.g., flash memory,used in such information processors, have been developed and produced.

[0005] The flash memory, which is one kind of non-volatile semiconductormemory device, can be made programmable by the user. Further, the flashmemory can be rewritten by electrically and collectively erasing thestored data, and then by programming. Therefore, the flash memories haveattracted considerable attention as a replacement for magnetic storagedevices because they are suitable for integration. Note, there is anecessity of improving redundant circuits, write circuits, andovererasing preventive measures for such a flash memory.

SUMMARY OF THE INVENTION

[0006] An object of a first aspect of the present invention is toprovide a semiconductor memory device for realizing effective word lineredundancy and stable write and verify operations in a semiconductormemory device, to improve the yield and performance of the semiconductormemory device.

[0007] An object of a second aspect of the present invention is toprovide a semiconductor memory device for reducing the size of thesemiconductor memory device, to improve the yield of large capacitysemiconductor memories and reduce the cost thereof.

[0008] An object of a third aspect of the present invention is toprovide a semiconductor memory device for carrying out a delivery testof a semiconductor memory device with “n” rewrite operations at themaximum, and taking into account deterioration due to an increase in thenumber of rewrite operations, to guarantee the maximum rewriteoperations N (N>n) for a user.

[0009] An object of a fourth aspect of the present invention is toprovide a semiconductor memory device that is capable of supplying awrite drain voltage that is not influenced by the threshold voltage of awrite voltage supply transistor, thereby correctly writing data to amemory cell even with a low write voltage.

[0010] An object of a fifth aspect of the present invention is toprovide a semiconductor memory device that correctly reads data even ifthere is an overerased cell transistor.

[0011] An object of a sixth aspect of the present invention is toprovide a semiconductor memory device that correctly reads data bysaving overerased cell transistors.

[0012] An object of a seventh aspect of the present invention is toprovide a semiconductor memory device capable of simultaneously erasinga plurality of blocks of memory cells and easily verifying the erasedblocks.

[0013] An object of an eighth aspect of the present invention is toprovide a semiconductor memory device employing two power sources thatare easy to use and operable like a single power source.

[0014] An object of a ninth aspect of the semiconductior memory deviceof the present invention is to provide an inexpensive decoder circuitthat solves the problems of the prior art. The decoder circuit of thepresent invention is simple and compact to achieve the full selectionand nonselection of word or bit lines in a test mode. This decodercircuit is suitable for high integration.

[0015] According to a first aspect of the present invention, there isprovided a semiconductor memory device comprising 2^(n) word lines, aplurality of bit lines, a plurality of nonvolatile memory cells eachformed of a MIS transistor disposed at each intersection of the wordlines and the bit lines, and a threshold voltage of the MIS transistorbeing externally electrically controllable, a write circuit for writingdata to a memory cell located at an intersection of selected ones of theword lines and the bit lines, a sense amplifier for reading data out ofthe memory cells, a first unit for simultaneously selecting a block of2^(m) (n>m) word lines among the 2^(n) word lines, and a second unit fornot selecting a block of 2^(k) (m>k) word lines among the 2^(m) wordlines, the second unit not selecting the block of 2^(k) word lines andselecting a block of 2^(k) word lines prepared outside the 2^(n) wordlines when any one of the 2^(k) word lines among the 2^(m) word lines isdefective.

[0016] The selected word lines may receive a negative voltage, and theunselected word lines receive a zero or positive voltage. The block of2^(n) word lines may form a real cell block, the block of 2^(m) wordlines may form an erase block, and the block of 2^(k) word lines outsidethe block of 2^(n) word lines may form a redundant cell block.

[0017] Further, according to a first aspect of the present invention,there is provided a semiconductor memory device comprising 2^(n) wordlines, a plurality of bit lines, a plurality of nonvolatile memory cellseach formed of a MIS transistor disposed at each intersection of theword lines and the bit lines, and a threshold voltage of the MIStransistor being externally electrically controllable, a write circuitfor writing data to a memory cell located at an intersection of selectedones of the word lines and the bit lines, a sense amplifier for readingdata out of the memory cells, a first unit for simultaneously selectinga block of 2^(m) (n>m) word lines among the 2^(n) word lines, and asecond unit for not selecting a block of 2^(k) (m>k) word lines amongthe 2^(m) word lines, data being written to any memory cell transistor,which is contained in the 2^(k) word lines and whose threshold voltageis lower than the potential of an unselected word line, so that thethreshold voltage of the memory cell transistor exceeds the potential ofthe unselected word line, and a block of 2^(k) word lines preparedoutside the 2^(n) word lines being used as redundant word lines.

[0018] Further, according to a first aspect of the present invention,there is also provided a semiconductor memory device comprising aplurality of word lines, a plurality of bit lines, a plurality ofnonvolatile memory cells each formed of a MIS transistor disposed ateach intersection of the word lines and the bit lines, and a thresholdvoltage of the MIS transistor being externally electricallycontrollable, a write circuit for writing data to a memory cell locatedat an intersection of selected ones of the word lines and the bit lines,and a sense amplifier for reading data out of the memory cells, whereineach word line is controlled such that a drain current of a memory celltransistor connected to the word line is lower than a channel currentthereof, when writing data to the cell transistor to increase thethreshold voltage of the memory cell transistor to be higher than thepotential of an unselected word line.

[0019] Each word line may be controlled by applying a signal inaccordance with a pulse signal.

[0020] Furthermore, according to a first aspect of the presentinvention, there is also provided a semiconductor memory devicecomprising a plurality of word lines, a plurality of bit lines, aplurality of nonvolatile memory cells each formed of a MIS transistordisposed at each intersection of the word lines and the bit lines, and athreshold voltage of the MIS transistor being externally electricallycontrollable, a write circuit for writing data to a memory cell locatedat an intersection of selected ones of the word lines and the bit lines,and a sense amplifier for reading data out of the memory cells, whereinan output current of the sense amplifier is changed according to acombination of ON states of two load transistors having differentcapacities, to realize a normal data read operation, an erase verifyoperation, and a write verify operation.

[0021] A reference voltage may be increased to provide a word line witha voltage, which is used to carry out the write verify or erase verifyoperations on any cell transistor connected to the word line. P-channeltype and n-channel type transistors fabricated in the same process maybe connected in series like diodes to provide a word line with a voltagewhich is used to carry out the write verify or erase verify operationson any cell transistor connected to the word line.

[0022] The semiconductor memory device may be constituted by a flashmemory.

[0023] According to a second aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of realmemory cells divided into blocks, a plurality of redundant memory cellsto be replaced with defective ones of the real memory cells, a pluralityof defective address specifying units for specifying defective addressesof the respective blocks of the real memory cells, and an addresscomparing unit shared by the defective address specifying unit, forcomparing the defective addresses with addresses in the blocks of thereal memory cells.

[0024] Further, according to a second aspect of the present invention,there is also provided a semiconductor memory device comprising a realcell array having a plurality of memory cells, and a plurality ofredundant cells to be replaced with defective memory cells of the realcell array, a redundant information storing cell array for writing datato a defective address according to an externally provided address, acell selection circuit for selecting the redundant information storingcell array according to the externally provided address, and a readcircuit for reading an output of the redundant information storing cellarray selected by the cell selection circuit, and providing a redundancysignal.

[0025] According to a third aspect of the present invention, there isprovided a semiconductor memory device comprising electrically erasablenonvolatile memory cells to and from which data is automatically writtenand erased according to an internal algorithm incorporated in thesemiconductor memory device, wherein the allowable value of write orerase operations is carried out according to the internal algorithmbeing variable.

[0026] According to a fourth aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of wordlines, a plurality of bit lines, a plurality of memory cells each formedof a MIS transistor disposed at each intersection of the word lines andthe bit lines, and a threshold voltage of the MIS transistor beingexternally electrically controllable, and a write voltage supplytransistor for supplying a write voltage to a drain of the memory cell,wherein the write voltage supply transistor is formed of a p-channeltype MIS transistor, which effectively applies the write voltage to thedrain of the memory cell.

[0027] Further, according to a fourth aspect of the present invention,there is also provided a semiconductor memory device comprising aplurality of word lines, a plurality of bit lines, a plurality of memorycells each formed of a MIS transistor disposed at each intersection ofthe word lines and the bit lines, and a threshold voltage of the MIStransistor being externally electrically controllable, and a writevoltage supply transistor for supplying a write voltage to a drain ofthe memory cell, wherein the write voltage supply transistor is formedof an n-channel type MIS transistor, and the semiconductor memory devicecomprises a step-up unit being disposed to increase a gate voltage ofthe write voltage supply transistor at least up to a sum of the writevoltage and a threshold voltage of the write voltage supply transistor.

[0028] According to a fifth aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of wordlines, a plurality of bit lines, a memory cell array including aplurality of memory cells each formed of a MIS transistor disposed ateach intersection of the word lines and the bit lines, a thresholdvoltage of the MIS transistor being externally electrically controllableaccording to charges to be injected to a floating gate thereof, and thefloating gates of the MIS transistors being simultaneously discharged tocollectively erase the memory cells, a first power source for applying anormal selection voltage to a selected word line to select memory cellsconnected to the word line, when reading data, and a second power sourcefor establishing an unselected state on unselected word lines includingmemory cells that have been overerased by the collective erasing, whenreading data.

[0029] Further, according to a fifth aspect of the present invention,there is also provided a semiconductor memory device comprising aplurality of word lines, a plurality of bit lines, a memory cell arrayincluding a plurality of memory cells each formed of a MIS transistordisposed at each intersection of the word lines and the bit lines, athreshold voltage of the MIS transistor being externally electricallycontrollable according to charges to be injected to a floating gatethereof, and the floating gates of the MIS transistors beingsimultaneously discharged to collectively erase the memory cells, afirst row decoder for applying a normal voltage to a selected word lineto select memory cells connected to the word line, when reading data,and a second row decoder for applying a predetermined source voltage tothe source of each memory cell connected to the selected word line, andapplying an unselected state establishing voltage to the sources ofmemory cells, including those overerased by the collective erasing,connected to unselected word lines, when reading data.

[0030] According to a sixth aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of wordlines, a plurality of bit lines, a memory cell array including aplurality of memory cells each formed of a MIS transistor disposed ateach intersection of the word lines and the bit lines, a thresholdvoltage of the MIS transistor being externally electrically controllableaccording to charges to be injected to a floating gate thereof, and thefloating gates of the MIS transistors being simultaneously discharged tocollectively erase the memory cells, wherein a method of savingovererased memory cells of the semiconductor memory device detectsmemory cells that have been overerased by the collective erasing, andwriting data to the overerased memory cells, thereby saving theovererased memory cells.

[0031] Further, according to a sixth aspect of the present invention,there is also provided a semiconductor memory device comprising aplurality of word lines, a plurality of bit lines, a memory cell arrayincluding a plurality of memory cells each formed of a MIS transistordisposed at each intersection of the word lines and the bit lines, and athreshold voltage of the MIS transistor being externally electricallycontrollable according to charges to be injected to a floating gatethereof, a write-before-erase unit for writing all memory cells of thememory cell array before erasing them, an erase unit for erasing all ofthe written memory cells by the write-before-erase unit and forverifying the erasing, an overerased cell detecting unit for detectingovererased memory cells among the erased and verified memory cells bythe erase unit, and an overerased cell saving unit for writing theovererased memory cells detected by the overerased cell detecting unit,thereby saving the overerased memory cells.

[0032] According to a seventh aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of wordlines, a plurality of bit lines, and a plurality of nonvolatile memorycells each formed of a MIS transistor disposed at each intersection ofthe word lines and the bit lines, and a threshold voltage of the MIStransistor being externally electrically controllable, wherein thenonvolatile memory cells areg divided into a plurality of cell blocks tobe selected according to a block selection signal provided by a blockaddress buffer, each of the cell blocks has a data erasing unit and alatching unit for latching the block selection signal, and thereby dataof the cell blocks that have latched the block selection signal aresimultaneously erased.

[0033] According to an eighth aspect of the present invention, there isprovided a semiconductor memory device comprising a first terminal forreceiving a normal voltage, a second terminal for receiving a highvoltage from a high-voltage supply unit, and the high voltage beingrequired to write or erase data and higher than the normal voltagerequired to read data, a third terminal for providing the high-voltagesupply unit with a control signal that controls the supply of the highvoltage.

[0034] Further, according to an eighth aspect of the present invention,there is also provided a semiconductor memory device comprising astep-up circuit for supplying a high voltage that is required to writeor erase data and higher than a normal voltage required to read data, acommand determination unit that determines whether or not an operationspecified by an input command to the semiconductor memory devicerequires the high voltage, and provides a control signal to start thesupply of the high voltage if the operation requires the high voltage,and if not, a control signal to stop the high voltage.

[0035] Furthermore, according to an eighth aspect of the presentinvention, there is also provided a computer system having asemiconductor memory device as a part of a storage unit and a step-upcircuit for generating a high voltage required to write and erase datato and from the semiconductor memory device, wherein the computer systemcomprises a control unit for automatically generating a control signalto control the step-up circuit, in response to an access operation tothe semiconductor memory device.

[0036] According to a ninth aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of wordlines, a plurality of bit lines, a plurality of memory cells disposed ateach intersection of the word lines and the bit lines, and a decodercircuit for selecting the memory cell according to an address signal ina normal decoding function and for carrying out a full selectionoperation or a nonselection operation of the word lines or the bit linesin a test function, and an output row or a decoding row connected to afirst power source and a second power source, the first power sourcesupplying a high voltage, and the second power source supplying areference voltage or the high voltage in response to a control signal.

[0037] Further, according to a ninth aspect of the present invention,there is also provided a semiconductor memory device comprising aplurality of word lines, a plurality of bit lines, a plurality of memorycells disposed at each intersection of the word lines and the bit lines,a decoder circuit for selecting the memory cell according to an addresssignal in a normal decoding function and for carrying out a fullselection operation or a nonselection operation of the word lines or thebit lines in a test function, and an output row or a decoding rowconnected to a first power source and a second power source, the firstpower source supplying a reference voltage, and the second power sourcesupplying a reference voltage or the high voltage in response to acontrol signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will be more clearly understood from thedescription of the preferred embodiments as set forth below withreference to the accompanying drawings, wherein:

[0039]FIG. 1 is diagram for explaining the operation of a memory cellused for a semiconductor memory device related to the first aspect ofthe present invention;

[0040]FIG. 2 is a block diagram showing a conventional semiconductormemory device related to the first aspect of the present invention;

[0041]FIG. 3 is a circuit diagram showing a column address buffer of thesemiconductor memory device of FIG. 2;

[0042]FIG. 4 is a circuit diagram showing a row address buffer of thesemiconductor memory device of FIG. 2;

[0043]FIG. 5 is a circuit diagram showing a row decoder of thesemiconductor memory device of FIG. 2;

[0044]FIG. 6 is a circuit diagram showing a column decoder of thesemiconductor memory device of FIG. 2;

[0045]FIG. 7 is a circuit diagram showing a write circuit of thesemiconductor memory device of FIG. 2;

[0046]FIG. 8 is a circuit diagram showing a source voltage supplycircuit of the semiconductor memory device of FIG. 2;

[0047]FIG. 9 is a circuit diagram showing a sense amplifier of thesemiconductor memory device of FIG. 2;

[0048]FIG. 10 is a diagram showing an example of write characteristicscurves of the semiconductor memory device of FIG. 2;

[0049]FIG. 11 is a circuit block diagram showing an embodiment of asemiconductor memory device according to the first aspect of the presentinvention;

[0050]FIG. 12 is a circuit diagram showing a row address buffer of thesemiconductor memory device of FIG. 11;

[0051]FIG. 13 is a circuit diagram showing a row decoder of thesemiconductor memory device of FIG. 11;

[0052]FIG. 14 is a circuit diagram showing a coincidence circuit of thesemiconductor memory device of FIG. 11;

[0053]FIG. 15 is a circuit diagram showing an essential part of a rowdecoder of the semiconductor memory device of FIG. 11;

[0054]FIG. 16 is a diagram showing a waveform of a signal applied to thecircuit of FIG. 15;

[0055]FIG. 17 is a circuit diagram showing a verify voltage generator ofthe semiconductor memory device of FIG. 11;

[0056]FIG. 18 is a circuit diagram showing a sense amplifier of thesemiconductor memory device of FIG. 11;

[0057]FIG. 19 is a circuit diagram showing a logic circuit for producinga control signal to the sense amplifier of FIG. 18;

[0058]FIG. 20 is a circuit block diagram showing a redundant circuit ofa conventional semiconductor memory device related to the second aspectof the present invention;

[0059]FIG. 21 is a circuit block diagram showing an arrangement of theconventional redundant circuits of FIG. 20;

[0060]FIG. 22 is a block diagram showing a semiconductor memory deviceemploying the conventional redundant circuit of FIG. 20;

[0061]FIG. 23 is a circuit block diagram showing an embodiment of aredundant circuit for the semiconductor memory device according to thesecond aspect of the present invention;

[0062]FIG. 24 is a diagram showing real cells and redundant cells of asemiconductor memory device employing the redundant circuit of FIG. 23;

[0063]FIG. 25 is a block diagram showing a semiconductor memory deviceemploying the redundant circuit of FIG. 23;

[0064]FIG. 26 is a circuit block diagram showing another embodiment of aredundant circuit for the semiconductor memory device according to thesecond aspect of the present invention;

[0065]FIG. 27 is a circuit block diagram showing still anotherembodiment of a redundant circuit for the semiconductor memory deviceaccording to the second aspect of the present invention;

[0066]FIG. 28 is a block diagram showing a semiconductor memory deviceemploying the redundant circuit of FIG. 27;

[0067]FIG. 29 is a flowchart showing an example of an internal writealgorithm for a semiconductor memory device according to the thirdaspect of the present invention;

[0068]FIG. 30 is a block diagram showing an embodiment of asemiconductor memory device according to the third aspect of the presentinvention;

[0069]FIG. 31 is a circuit diagram showing an essential part of thesemiconductor memory device of FIG. 30;

[0070]FIG. 32 is a timing chart for explaining the operation of thecircuit of FIG. 31;

[0071]FIG. 33 is diagram for explaining the operation of a memory cellof a semiconductor memory device according to the fourth aspect of thepresent invention;

[0072]FIG. 34 is a circuit block diagram showing an example of aconventional semiconductor memory device related to the fourth aspect ofthe present invention;

[0073]FIG. 35 is a circuit block diagram showing an embodiment of asemiconductor memory device according to the fourth aspect of thepresent invention;

[0074]FIG. 36 is a circuit diagram showing an essential part of anotherembodiment of a semiconductor memory device according to the fourthaspect of the present invention;

[0075]FIG. 37 is a circuit block diagram showing a conventionalsemiconductor memory device related to the fifth aspect of the presentinvention;

[0076]FIG. 38 is a circuit diagram showing a row decoder of thesemiconductor memory device of FIG. 37;

[0077]FIG. 39 is a circuit diagram showing a column decoder of thesemiconductor memory device of FIG. 37;

[0078]FIG. 40 is a circuit diagram showing bit line transfer gates ofthe column decoder of FIG. 39;

[0079]FIG. 41 is a circuit block diagram showing an embodiment of asemiconductor memory device according to the fifth aspect of the presentinvention;

[0080]FIG. 42 is a circuit diagram showing a row decoder of thesemiconductor memory device of FIG. 41;

[0081]FIG. 43 is a circuit block diagram showing another embodiment of asemiconductor memory device according to the fifth aspect of the presentinvention;

[0082]FIG. 44 is a circuit block diagram showing first and second rowdecoders of the semiconductor memory device of FIG. 43;

[0083]FIG. 45 is a circuit diagram showing a part of the second rowdecoder of FIG. 44;

[0084]FIG. 46 is a circuit block diagram showing an essential part of anembodiment of a semiconductor memory device according to the sixthaspect of the present invention;

[0085]FIG. 47 is a circuit diagram showing a sense amplifier of thesemiconductor memory device of FIG. 46;

[0086]FIG. 48 is a block diagram schematically showing a systememploying the semiconductor memory device according to the sixth aspectof the present invention;

[0087]FIG. 49 is a flowchart for explaining an example of a processcarried out by the semiconductor memory device of the sixth aspect ofthe present invention;

[0088]FIG. 50 is diagram for explaining the operation of a memory cellof a semiconductor memory device according to the seventh aspect of thepresent invention;

[0089]FIG. 51 is a circuit diagram showing a conventional semiconductormemory device related to a semiconductor memory device according to theseventh aspect of the present invention;

[0090]FIG. 52, consisting of FIGS. 52A and 52B, is a circuit blockdiagram showing an embodiment of a semiconductor memory device accordingto the seventh aspect of the present invention;

[0091]FIG. 53 is a circuit diagram showing a source voltage supplycircuit of the semiconductor memory device of FIG. 52;

[0092]FIG. 54 is a circuit diagram showing an expected value storagecircuit of the semiconductor memory device of FIG. 52;

[0093]FIG. 55 is a circuit diagram showing a coincidence circuit of thesemiconductor memory device of FIG. 52;

[0094]FIG. 56, consisting of FIGS. 56A and 56B, is a circuit blockdiagram showing another embodiment of a semiconductor memory deviceaccording to the seventh aspect of the present invention;

[0095]FIG. 57 is a circuit diagram showing an expected value generatorof the semiconductor memory device of FIG. 56;

[0096]FIG. 58, consisting of FIGS. 58A and 58B, is a circuit blockdiagram showing still another embodiment of a semiconductor memorydevice according to the seventh aspect of the present invention;

[0097]FIG. 59 is a circuit diagram showing a block selection signalstorage circuit of the semiconductor memory device of FIG. 58;

[0098]FIG. 60 is a circuit diagram showing a write circuit of thesemiconductor memory device of FIG. 58;

[0099]FIG. 61 is a circuit diagram showing a data inversion circuit ofthe semiconductor memory device of FIG. 58;

[0100]FIG. 62 is a block diagram showing a conventional system using asemiconductor memory device related to the eighth aspect of the presentinvention;

[0101]FIG. 63 is a flowchart showing an example of the control operationfor controlling the system of FIG. 62;

[0102]FIG. 64 is a diagram for explaining the principle of asemiconductor memory device according to the eighth aspect of thepresent invention;

[0103]FIG. 65 is a flowchart showing the steps of processing a highvoltage requiring operation in a system employing a semiconductor memorydevice according to the eighth aspect of the present invention;

[0104]FIG. 66 is a block diagram showing a total system using asemiconductor memory device according to the eighth aspect of thepresent invention;

[0105]FIG. 67 is a block diagram showing an embodiment of asemiconductor memory device according to the eighth aspect of thepresent invention;

[0106]FIG. 68 is a flowchart showing an example of the control operationfor controlling the system of FIG. 66;

[0107]FIG. 69 is a circuit block diagram showing a voltage test circuitshown in FIG. 67;

[0108]FIG. 70 is a circuit block diagram for explaining the start of anext operation a predetermined time after the transmission of a controlsignal;

[0109]FIG. 71 is a diagram schematically showing an example of a DC-DCconverter;

[0110]FIG. 72 is a diagram schematically showing another example of aDC-DC converter;

[0111]FIG. 73 is a block diagram showing a semiconductor memory devicehaving DC-DC converter according to the eighth aspect of the presentinvention;

[0112]FIG. 74 is a circuit diagram showing a conventional decodercircuit used in a semiconductor memory device related to the ninthaspect of the present invention;

[0113]FIG. 75 is a circuit diagram showing another conventional decodercircuit used in a semiconductor memory device related to the ninthaspect of the present invention;

[0114]FIG. 76 is a circuit diagram showing still another conventionaldecoder circuit used in a semiconductor memory device related to theninth aspect of the present invention;

[0115]FIG. 77 is a circuit diagram showing still another conventionaldecoder circuit used in a semiconductor memory device related to theninth aspect of the present invention;

[0116]FIG. 78 is a circuit block diagram showing an embodiment of adecoder circuit used in a semiconductor memory device according to theninth aspect of the present invention;

[0117]FIG. 79 is a circuit block diagram showing another embodiment of adecoder circuit used in a semiconductor memory device according to theninth aspect of the present invention;

[0118]FIG. 80A is a circuit block diagram showing still anotherembodiment of a decoder circuit used in a semiconductor memory deviceaccording to the ninth aspect of the present invention;

[0119]FIG. 80B is a modification of the decoder circuit shown in FIG.80A;

[0120]FIG. 81A is a circuit block diagram showing still anotherembodiment of a decoder circuit used in a semiconductor memory deviceaccording to the ninth aspect of the present invention;

[0121]FIG. 81B is a modification of the decoder circuit shown in FIG.81A;

[0122]FIG. 82 is a circuit block diagram showing an example of asemiconductor memory device using a decoder circuit according to theninth aspect of the present invention;

[0123]FIGS. 83A and 83B are circuit diagrams showing the details of thedecoder circuit of FIG. 78;

[0124]FIGS. 84A and 84B are circuit diagrams showing the details of thedecoder circuit of FIG. 79;

[0125]FIGS. 85A and 85B are circuit diagrams showing the details of thedecoder circuit of FIG. 80A;

[0126]FIGS. 86A and 86B are circuit diagrams showing the details of thedecoder circuit of FIG. 81A;

[0127]FIG. 87 is a circuit diagram showing a power supply circuit forthe decoder used in a semiconductor memory device according to the ninthaspect of the present invention;

[0128]FIG. 88 is a circuit diagram showing a conventional test modesignal detector of a semiconductor device related to the ninth aspect ofthe present invention;

[0129]FIG. 89 is a timing chart for explaining the problems of thedetector of FIG. 88;

[0130]FIG. 90 is a circuit diagram showing a test mode signal detectorof a semiconductor device according to the ninth aspect of the presentinvention;

[0131]FIG. 91 is a timing chart of the detector of FIG. 90; and

[0132]FIG. 92 shows a decoder circuit shown in FIG. 85B, using thedetectors 930 shown in FIG. 90.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0133] For a better understanding of the preferred embodiments, theproblems of the related art will be explained, with reference to FIGS. 1to 10.

[0134]FIG. 1 shows a memory cell (MC) of an electrically collectivelyerasable nonvolatile semiconductor memory device (a flash memory) forwhich a first aspect of the present invention is applied. The memorycell (cell transistor) MC has a floating gate FG. The floating gate FGis located between a source and a drain and is insulated from otherregions. A control gate CG is formed above the floating gate FG.

[0135] To write data to the cell transistor MC, a drain voltage Vdapplied to a drain DD is substantially equalized with a power sourcevoltage Vcc. A gate voltage Vg applied to the control gate CG is apositive high voltage (about +10 volts at the maximum). A source voltageVs applied to a source SS is zero. Electrons are injected from the drainDD to the floating gate FG, to write data “0” to the cell transistor MC.The drain voltage Vd may be a write voltage Vpp, if the voltage Vpp isavailable. The high gate voltage Vg may be the write voltage Vpp, or maybe produced by increasing the power source voltage Vcc.

[0136] To erase data from the cell transistor MC, the gate voltage Vg isset to a high negative value (about −10 volts at the lowest). The drainvoltage Vd is opened to put the drain DD in a floating state. The sourcevoltage Vs is equalized with the power source voltage Vcc. Electrons arepulled from the floating gate FG to the source SS, to erase data fromthe cell transistor MC, i.e., to write data “1” to the cell transistor.To read data from the cell transistor MC, the gate voltage Vg isequalized to the power source voltage Vcc, the drain voltage Vd to aboutone volt, and the source voltage Vs to zero. Then, it is checked to seewhether or not a drain current flows, to determine if the data stored inthe cell transistor MC is “1” or “0”.

[0137]FIG. 2 shows a conventional semiconductor memory device related tothe first aspect of the present invention. This semiconductor memorydevice has a row address buffer 111, a row decoder 112, a column addressbuffer 113, a column decoder 114, a data I/O buffer 115, a write circuit116, a sense amplifier 117, a negative voltage generator 118, and asource voltage supply circuit 119. The semiconductor memory deviceinvolves bit lines BLs, word lines WLs, a write control signal W thatwill be high level H when writing data, and an erase control signal Ethat will be high level H when erasing data.

[0138] When reading data from a memory cell (cell transistor) MC, a rowaddress is given to select a word line WL, and a column address toselect a bit line BL, thereby selecting the memory cell. The senseamplifier 117 senses whether or not a current flows through the selectedmemory cell, to determine whether data stored in the cell is “1” or “0”.

[0139] When writing data to a memory cell MC, the write control signal Wis set to high level H. The write circuit 116 provides a bus BUS with awrite voltage, and the column decoder 114 connects the bus BUS to agiven bit line BL. The row decoder 112 applies the write voltage to aselected word line WL. When erasing data from memory cells MCs, theerase control signal E is set to high level H. The source voltage supplycircuit 119 applies an erase voltage to source lines of the memory cellsMCs, and the column address buffer 113 puts the bit lines BLs in anunselected state. The row address buffer 111 simultaneously selects agiven number of word lines WLs, and the row decoder 112 sets theselected word lines WLs to low level L and the unselected word lines WLsto high level H. The negative voltage generator 118 applies a negativevoltage to the word lines WLs of low level L.

[0140] FIGS. 3 to 6 show examples of the column address buffer 113, rowaddress buffer 111, row decoder 112, column decoder 114 of thesemiconductor memory device of FIG. 2, respectively.

[0141] When reading data from a memory cell MC, the erase control signalE is at low level L. The column address buffer 113 of FIG. 3 and rowaddress buffer 111 of FIG. 4 provide positive and negative logic valueswith respect to an input address. The row decoder 112 of FIG. 5 involvesa signal φ that oscillates at a given frequency when erasing or writingdata, and a signal φ_(R) that is at high level H for some period of timeafter the reception of the input address.

[0142] When reading data, the write control signal W is at low level L.Transistors T₁ and T₃ of the row decoder 112 of FIG. 5 introduce thepower source voltage Vcc. According to an address input (an output ofthe row address buffer 111), a predetermined decoder is selected. Forexample, a node N₃ of FIG. 5 becomes high level H. If the signal φ_(R)provides a pulse of high level H in this state, nodes N₂ and N₄ arereset to zero. When the signal φ_(R) restores low level L, the node N₂is charged to the power source voltage Vcc. Due to the self-bootstrapeffect of transistors T₄ and T₇, the node N₄ is also charged to thelevel of the power source voltage Vcc. The column decoder 114 operatessimilarly to the row decoder 112. As a result, the power source voltageVcc is applied to a predetermined word line WL, and a predetermined bitline BL is connected to the sense amplifier 117.

[0143]FIGS. 7 and 8 show examples of the write circuit 116 and sourcevoltage supply circuit 119 of the semiconductor memory device of FIG. 2,respectively.

[0144] With the write control signal W of high level H, data of lowlevel L, and an inverted data signal /DATA of high level H, the writecircuit 116 of FIG. 7 increases the power source voltage Vcc to providethe bus BUS with a high voltage. With this high voltage, data is writtento a given cell transistor. The signal /DATA is transferred as a writesignal from the data I/O buffer 115 to the write circuit 116. Note, amark “/” denotes an inverted level or inverted signal, and thus, forexample, the signal /DATA denotes an inverted level of signal DATA.

[0145] When erasing data, the erase control signal E is raised to highlevel H. In the column address buffer 113 of FIG. 3, both outputs A and/A are set to low level L. These outputs A and /A are applied to thecolumn decoder 114 to put the columns (bit lines BLs) in an unselectedstate. Namely, the bit lines BLs are electrically disconnected fromevery node. In the row address buffer 111, the erase control signal E isapplied to “m” buffer elements among “n” buffer elements in total. As aresult, the row decoder 112 of FIG. 5 simultaneously selects 2 ^(m) wordlines. In the row decoder 112, the erase control signal E is at highlevel H, so that the node N₂ receives zero volts and the node N₅receives high level H. As a result, the selected word lines WLs are setto low level L, and the unselected word lines WLs are set to high levelH.

[0146] The negative voltage generator 118 provides an erase voltage tothe word lines WLs of low level L. The word lines WLs at high level Hare maintained at high level H because the potential of an output N₄ ofa NOR gate of FIG. 5 is always at low level L, and therefore, the signalφ is not transmitted to a capacitance element connected to the node N₄.At this time, the source voltage supply circuit 119 of FIG. 8 appliesthe power source voltage Vcc to the source SS of each cell transistorMC. As a result, the cell transistors of the 2^(m) word lines aresimultaneously erased.

[0147]FIG. 9 shows an example of the sense amplifier 117 of thesemiconductor memory device of FIG. 2.

[0148] An output of the sense amplifier 117 of FIG. 9 will be high levelH or low level L depending on whether or not the drain current of aselected cell transistor MC is larger than the allowable current of atransistor T₈. Transistors T₉, T₁₀, T₁₁, and T₁₂ form a bias circuitthat sets the potential of the bus BUS to about one volt.

[0149] When writing data, the write control signal W is set to highlevel H, and the signal φ is oscillated at a predetermined frequency.The transistors T₄ and T₅ provide the node N₁ with a write voltage.Similar to the read operation with the signal φ_(R) a pulse of highlevel H is applied to charge the node N₂ to the write voltage. The nodeN₄ is also charged to the same level as the node N₂ due to theself-bootstrap effect of the transistors T₄ and T₇. The column decoder114 operates in the same manner. Consequently, the write voltage issupplied to a selected word line WL, and a selected bit line BL isconnected to the write circuit 116.

[0150] As explained with reference to FIGS. 2 to 9, the semiconductormemory device (flash memory) of the related art collectively erases ablock containing a large number of memory cells, e.g., 512 kilobits. Ifthis large block includes a defective cell, the block as a whole must bereplaced with a block containing a large number of redundant memorycells. This sort of replacement deteriorates the efficiency ofredundancy. Namely, it is difficult for the related art to replace manydefective cells with a small number of redundant cells. If the memorycell MC11 of FIG. 1 is overerased, a current will always flow to the bitline BL1 through the memory cell MC11, to hinder correct read and writeoperations.

[0151]FIG. 10 shows the write characteristics of a semiconductor memorydevice (a flash memory).

[0152] The related art mentioned above provides a write drain voltage byincreasing the power source voltage Vcc. Meanwhile, the bit line drivingcapacity of the write circuit 116 is limited, so that the potential of abit line decreases when a large current is supplied to the bit line.When an overerased cell transistor is present, the write characteristiccurve of the overerased cell transistor may cross a load curve of thewrite circuit 116 in a write impossible region A, as indicated withcontinuous lines in FIG. 10. If this happens, a write operation will bedisabled. The write operation is enabled only between points D and B. Aword line voltage for an erase or write verify operation is usuallyprovided by dropping the external write voltage. Since the related artemploys no external write voltage, it is difficult for the related artto carry out the verify operation. If some cell transistors areovererased, it is difficult to restore normal operation by replacing theovererased cell transistors with spare cell transistors. Namely, thenormal operation will never resume by replacing word lines containingthe overerased cells with redundant word lines. In this case, theovererased cells may be rewritten to cancel the overerased state andrestore the normal operation. The overerased cells, however, pass alarger current around a point A of FIG. 10 to make such writingdifficult.

[0153] Below, the preferred embodiments of a semiconductor memory deviceaccording to the present invention will be explained, with reference tothe accompanying drawings.

[0154] First, a semiconductor memory device according to a first aspectof the present invention will be explained with reference to FIGS. 11 to19.

[0155]FIG. 11 shows the semiconductor memory device (flash memory)according to the first aspect of the present invention. This memorydiffers from the conventional semiconductor memory device of FIG. 2 inthat it additionally has a coincidence circuit 120 for comparing aninput address with a defective address, and a redundant row decoder 130.A row address buffer 101, a row decoder 102, a column address buffer103, a column decoder 104, a data I/O buffer 105, a write circuit 106, asense amplifier 107, a negative voltage generator 108, and a sourcevoltage supply circuit 109 of this embodiment correspond to the rowaddress buffer 111, row decoder 112, column address buffer 113, columndecoder 114, data I/O buffer 115, write circuit 116, sense amplifier117, negative voltage generator 118, and source voltage supply circuit119 of the related art of FIG. 2.

[0156] The operation of the embodiment will now be explained. Whenwriting or reading data to or from a memory cell (cell transistor) ofthe semiconductor memory device, the coincidence circuit 120 compares aninput address with each defective address stored therein. If they agreewith each other, the coincidence circuit 120 provides the row addressbuffer 101 and redundant row decoder 130 with an output signal to putthe row decoder 102 in an unselected state and the redundant row decoder130 in a selected state. This enables a redundant cell to be accessedinstead of a defective cell. When erasing data from the semiconductormemory device, an erase control signal E of high level H is provided tothe column address buffer 103, row address buffer 101, row decoder 102,coincidence circuit 120, and redundant row decoder 130.

[0157] If there is no defective cell (transistor) in a memory cellarray, i.e., if the redundancy is not used, a normal erase operation iscarried out as previously explained. A redundancy control signal REDfrom the coincidence circuit 120 indicates no redundancy.

[0158] If there is a defective cell, the coincidence circuit 120 storesthe address of the defective cell. In this embodiment, there are 2^(n)word lines in total, and 2^(m) word lines among the 2^(n) word linesform an erase block. The embodiment employs 2^(k) redundant (spare) wordlines. To write or read data, the number of bits of defective addressstorage elements in the coincidence circuit 120 must be “n−k.” Since anerase block includes 2^(m) word lines, the number of address bitsnecessary for selecting the erase block is “n−m.”When erasing data,“n−m” addresses are provided to select a block of 2^(m) word lines. Theinput addresses are compared with upper “n−m” address bits among the“n−k” address bits stored in the coincidence circuit 120. If they agreewith each other, it is determined that the erase block of 2^(k) wordlines includes a defective cell.

[0159] Address information representing the block of 2^(k) word linesincluding the defective cell among the block of 2^(m) word lines isgiven by the remaining “m−k” address bits stored in the coincidencecircuit 120. Consequently, the row decoder 102 sets the block of 2^(k)word lines specified with the “m−k” address bits among the block of2^(m) word lines to be unselected. When the coincidence of “n−m” addressbits is found in erasing 2^(m) word lines, the redundant row decoder 130is activated to erase the 2^(k) redundant word lines.

[0160] FIGS. 12 to 14 show essential parts of the row address buffer101, row decoder 102, and coincidence circuit 120 of the semiconductormemory device of FIG. 11.

[0161] In FIG. 12, there are “n” buffers in the row address buffer 101.Lower “m” buffers receive the erase control signal E to select 2^(m)word lines in an erase operation. Any one of the m buffers receives theredundancy control signal RED. If this signal is high level H in a writeor read operation, it means that an input address agrees with adefective address, and therefore, the word line WL is set to beunselected.

[0162] The redundancy control signal RED is an output signal of thecoincidence circuit 120 of FIG. 14. The coincidence circuit 120 includesaddress storage fuses used to specify a block of 2^(k) word lines amongthe 2^(n) word lines, and a fuse RUSE used to store the state of use ofredundancy. Since the erase control signal E is at low level L except inan erase operation, the redundancy control signal RED becomes high levelH only when all pieces of fuse information agree with input informationpieces. In the erase operation, the redundancy control signal REDbecomes high level H if upper “n−m” addresses A_(RBm+) _(¹) to A_(RBn)coincide with input addresses. Fuse data of “m−k” addresses A_(RBk+)_(¹) to A_(RBm) are provided directly to a NAND gate of FIG. 13. As aresult, a block of 2^(k) word lines among a block of 2^(m) word lines isset to be unselected. The redundancy control signal RED is also providedto the redundant row decoder 130. In this way, an optional block of2^(k) word lines in an erase block of 2^(m) word lines is replaceablewith a block of redundant (spare) word lines.

[0163] If a memory cell is overerased in the semiconductor memory device(flash memory), it deteriorates the yield. The bit lines of thesemiconductor memory device are shared by the redundant cells and realcells, and therefore, it is impossible to save the overerased cell byreplacing it with the redundant cell. For example, if a memory cell(cell transistor) MC₁₁ is overerased in FIG. 11, the cell MC₁₁ may bereplaced with a redundant cell MCR₁₁. In this case, however, theovererased cell MC₁₁ passes current even when a word line WL1 is at lowlevel L. If this happens, data “0” of a cell existing on the bit lineBL1 will not be correctly read. This problem is easily solved by writingdata “0” in the overerased cell MC₁₁ before replacing it with theredundant cell MCR₁₁. Namely, electrons are injected into the floatinggate of the cell MC₁₁ before replacing it with the redundant cell.

[0164] The overerasing of a memory cell will be explained with referenceto FIG. 10, which shows the write characteristics curve of asemiconductor memory device. The floating gate of an overerased cell ispositively charged to increase a current at a point A on the curve, thusdisabling data write. To solve this problem, it is necessary to controlthe gate level of any cell when writing data to the cell, such that acurrent flowing through the cell does not exceed the load curve of thewrite circuit 106 around the point A. This is easily achieved bycontinuously providing pulses to a corresponding word line WL during thedata write. When the word line WL receives the pulses, it surelyrealizes a curve C indicated with a dotted line in FIG. 10 when movingfrom low level L to high level H, or from high level H to low level L.Consequently, the data write is enabled irrespective of the condition ofthe floating gate.

[0165]FIG. 15 shows an essential part of an example of the row decoder102 of the semiconductor memory device of FIG. 11, and corresponds tothe input portion B of the row decoder 112 of the conventionalsemiconductor memory device of FIG. 5. A NOR gate of FIG. 15 receives asignal φW whose waveform is shown in FIG. 16. With this signal, thepotential of the node N₃ of FIG. 5 is continuously oscillated betweenzero and a write potential, to continuously provide pulses to a wordline WL. Instead of applying the pulses, it is possible to employ acircuit for applying an intermediate voltage to a word line. Generally,a write or erase operation of a semiconductor memory device is followedby a verify operation, which is achieved by applying a verify voltage toread data from a word line. The verify voltage must be constantirrespective of changes in environmental conditions of the semiconductormemory device such as a change in a power source voltage. Accordingly,it is practical to provide the verify voltage by increasing a referencepotential (Vss) of the semiconductor memory device.

[0166]FIG. 17 shows an example of a verify voltage generator 150 of thesemiconductor memory device of FIG. 11. This circuit generates a verifyvoltage applied to the node N₁ of the row decoder circuit 112 (102) ofFIG. 5.

[0167] The verify voltage generator 150 of FIG. 17 includes a clampcircuit 151, an oscillator 152, and a step-up circuit 153. The clampcircuit 151 has transistors T₁₃ and T₁₄ for determining a clamp voltage.These transistors are p-channel type and n-channel type MOS (MIS)transistors connected in series as diodes. According to a CMOS process,the channel regions of transistors are simultaneously fabricated tocancel fluctuations in the thresholds of the transistors. This resultsin stabilizing the clamp voltage.

[0168] An n-channel type MOS transistor T₁₅ has a threshold of aboutzero volts. This transistor provides the oscillator 152 with the clampvoltage. The step-up circuit 153 operates according to a low powersource voltage (ground voltage) Vss. A verify voltage at the node N₁,therefore, is stable regardless of the power source voltage. Erase andwrite verify operations are carried out with different voltages.Different clamp voltages are easily produced by changing the number oftransistors (T₁₃, T₁₄, . . . ) of the clamp circuit 151. A signal VRbecomes high level H when carrying out the verify operation. The verifyoperation may be carried out by changing a decision current of the senseamplifier.

[0169]FIG. 18 shows an example of the sense amplifier 107 of the flashmemory of FIG. 11. The sense amplifier 107 has p-channel typetransistors T_(L) _(¹) and T_(L) _(²) serving as load transistors. Thecurrent supply capacities of these transistors are T_(L) _(¹) >T_(L)_(²) . The flash memory has three modes, i.e., an erase verify mode, anormal read mode, and a write verify mode. The total capacity of theload transistors for these modes must be the erase verify mode>thenormal read mode>the write verify mode. The circuit of FIG. 18 realizesthis relationship with V_(R) _(¹) =V_(R) _(²) =L for the erase verifymode, V_(R) _(¹) =L and V_(R) _(²) =H for the normal read mode, andV_(R) _(¹) =H and V_(R) _(²) =L for the write verify mode.

[0170]FIG. 19 shows an example of a logic circuit for providing thecontrol signals V_(R) _(¹) and V_(R) _(²) to the sense amplifier of FIG.18. The logic circuit of FIG. 19 involves a write verify signal WV andan erase verify signal EV. This circuit is advantageous in simplifyingthe power source of the row decoder when producing the verify voltage.The load control logic circuit is applicable for the erase verifyoperation of the flash memory.

[0171] A semiconductor memory device according to a second aspect of thepresent invention will be explained with reference to FIGS. 20 to 28.

[0172]FIG. 20 shows a redundant circuit 210 of a conventionalsemiconductor memory device related to the second aspect of the presentinvention. The redundant circuit 210 has a fuse 211 serving as defectiveaddress specifying means for storing a defective address. The fuse 211is connected or disconnected to provide high (H) or low (L) level. Acomparator 214 compares the information of the fuse 211 with an externalinput address, and if they agree with each other, provides an addresscoincidence signal of, for example, high level H.

[0173]FIG. 21 shows a redundant arrangement 2100, which contains aplurality of the conventional redundant circuits of FIG. 20. Outputs ofthe redundant circuits 210 are passed through a NAND gate and aninverter, to provide a redundancy signal. Only when each input addressagrees with the information of the fuse 211 in every redundant circuit210, is a redundancy signal of high level H provided to read data fromredundant cells.

[0174]FIG. 22 shows a semiconductor memory device employing theconventional redundant arrangement 2100 of FIG. 21. When the redundantarrangement 2100 provides a redundancy signal, a real cell selector 217prohibits reading of a real cell 218, and a redundant cell selector 215reads a redundant cell 216, thereby replacing a defective real cell witha redundant cell. A data read circuit 219 reads data out of a selectedone of the redundant cell 216 and real cell 218.

[0175] According to the prior art of FIGS. 20 to 22, each fuse (211)requires one address comparator (214). When many defective memory cellsmust be replaced with redundant cells, a corresponding numbers of fusesand address comparators must be prepared. This results in increase inthe size and cost of a semiconductor chip.

[0176] In this way, the conventional redundant technique has a problemof increasing the size and cost of a semiconductor memory device whenthe numbers of memory cells and redundant cells increase.

[0177] An object of a second aspect of the present invention is toprovide a semiconductor memory device for reducing the size of thesemiconductor memory device, to improve the yield of large capacitysemiconductor memories and reduce the cost thereof.

[0178]FIG. 23 shows a redundant circuit of a semiconductor memory deviceaccording to the second aspect of the present invention. This redundantcircuit 200 employs a transistor T_(A), a fuse 201A, a transistor T_(B),and a fuse 201B in place of the fuse 211 of the conventional redundantcircuit 210 of FIG. 20. The fuses 201A and 201B are selected accordingto the logic of an external input address An (/An, an inversion of theaddress An). The address An (/An) is an upper address for selecting oneof the blocks of real cells. The fuses 201A and 201B of the redundantcircuit 200 share an address comparator 204, to reduce the total numberof the address comparators 204 and the size and cost of thesemiconductor memory device.

[0179]FIG. 24 shows an arrangement of real cells 208 and redundant cells206 in the semiconductor memory device employing the redundant circuitof FIG. 23. The real cells 208 are divided into a first real cell block208A to be selected with a block selection address An of low level L,and a second real cell block 208B to be selected with a block selectionaddress An of high level H and an inverted address /An of low level L.The redundant cells 206 are divided into a first redundant cell block206A to be replaced with some cells of the first real cell block 208A,and a second redundant cell block 206B to be replaced with some cells ofthe second real cell block 208B. When an address (An−1, An−2, . . . ,etc) that is common for the real cell blocks 206A and 206B includes adefective cell, a predetermined number of real cells including thedefective cell among the block specified by the block address An arereplaced with the redundant cells.

[0180]FIG. 25 shows a semiconductor memory device employing theredundant circuit of FIG. 23 according to the present invention. Whenthe redundant circuit 200 provides a redundancy signal, a real cellselector 207 prohibits the reading of a real cell 208, and a redundantcell selector 205 reads a redundant cell 206. Unlike the semiconductormemory device of FIG. 22, the semiconductor memory device of FIG. 25provides an address (a block selection address An) input to theredundant cell selector 205 as well, which selects one of the redundantcells 206A and 206B corresponding to the fuses 201A and 201B of theredundant circuit 200 in response to a logic of the address An. Namely,the redundant cell selector 205 receives the block address An andselects one of the redundant cells 206A and 206B corresponding to one ofthe real cell blocks 208A and 208B that is selected according to theblock address An. In FIG. 25, a data read circuit 209 reads data out ofa selected one of the redundant cell 206 and real cell 208.

[0181] As explained above, the address comparator 204 is shared by theredundant cell blocks 206A and 206B when replacing a defective real cellwith a redundant cell. In the above explanation, the block selectionaddress An is of one bit for selecting one of the two real cell blocks.The block selection address may involve two bits An and An−1 to selectone of the four real cell blocks and one of the four redundant cellblocks.

[0182]FIG. 26 shows another redundant circuit 200′ for a semiconductormemory device according to the second aspect of the present invention.The redundant circuit 200′ includes a cell selector 220, redundantinformation storing cell arrays 221 and 223, and read circuits 222 and224.

[0183] The redundant circuit 200′ has the two redundant informationstoring cell arrays 221 and 223 and the two read circuits 222 and 224.

[0184] Each of the redundant information storing cell arrays 221 and 223includes nonvolatile memory cell transistors such as EPROMs and is usedto write data to defective addresses according to external inputaddresses. The cell selector 220 selects one of the cell arrays 221 and223 according to an address input. The read circuits 222 and 224 providefour redundancy signals through AND gates 225A, 225B, 225C, and 225D,and through inverters 226A and 226B. This embodiment provides two-bitdata from the two cell arrays 221 and 223 in parallel, so that fourdefective cells can be replaced with redundant cells. It is alsopossible to provide data of three bits or more in parallel.

[0185]FIG. 27 shows still another redundant circuit 200″ according tothe second aspect of the present invention, and FIG. 28 shows asemiconductor memory device employing the redundant circuit of FIG. 27.

[0186] Unlike the redundant circuit 200′ of FIG. 26 that provides aplurality of bits (two bits) in parallel, the embodiment of FIG. 27provides a single bit to replace a plurality of defective real cells 208with redundant cells 206.

[0187] The redundant circuit 200″ reads a single bit according to anaddress input, and according to a logic of the bit, provides aredundancy signal. In the semiconductor memory device of FIG. 28, theredundant circuit 200″ provides a redundancy signal to a redundant cellselector 205′ and to a real cell selector 207′. A part (a real cellblock selecting address An) of an address input is also supplied to theselectors 205′ and 207′, to determine redundant cells to be replacedwith defective real cells.

[0188] A semiconductor memory device according to a third aspect of thepresent invention will be explained with reference to FIGS. 29 to 32.

[0189] Some electrically erasable programmable nonvolatile semiconductormemories such as flash memories have an internal algorithm forautomatically writing or erasing data.

[0190] The writing or erasing of data to a flash memory is carried outby applying a write pulse and by executing a read operation (a verifyoperation). If the read operation determines that the writing isinsufficient, the write pulse is again applied to repeat the writing orerasing. The maximum number of write pulses applied, i.e., the number ofverify operations is prescribed in specifications. These write, erase,and verify operations are externally controlled.

[0191] Some flash memories incorporate an algorithm for automaticallycarrying out the write or erase operation. A user of this kind of flashmemories is usually provided with the allowable number of write or eraseoperations.

[0192] Delivery tests of the flash memories sometimes require repetitiverewrite operations. This results in deteriorating the number of write orerase operations guaranteed for a user. In this case, the flash memoriesthat have passed the delivery test may fail on the user side beforereaching the guaranteed number of write or erase operations.

[0193] An object of a third aspect of the present invention is toprovide a semiconductor memory device for carrying out a delivery testof a semiconductor memory device with “n” rewrite operations at themaximum, and taking into account deterioration due to an increase in thenumber of rewrite operations, to guarantee the maximum rewriteoperations N (N>n) for a user.

[0194]FIG. 29 is a flowchart showing an algorithm to be stored in asemiconductor memory device according to the third aspect of the presentinvention.

[0195] After the start of a write process, step S301 applies a writepulse. Step S302 carries out a verify operation. Namely, step S302 readswritten data and determines whether or not the data has beensufficiently written. If the result is sufficient, the write processends. If it is insufficient, step S303 checks to see whether or not thenumber of applied write pulses has reached a predetermined number N. Ifit is NO, steps S301 and S302 are repeated. If it is YES, the writeprocess aborts. Namely, it is determined that the data has not beencorrectly written to a cell transistor after the application of N writepulses.

[0196] In this way, the third aspect of the present invention carriesout a delivery test of a semiconductor memory device with “n” writepulses at the maximum, which is smaller than an allowable number “N”.Namely, the delivery test is carried out under severer conditions thannormal conditions, thereby guaranteeing the allowable number N for auser.

[0197]FIG. 30 shows a semiconductor memory device according to the thirdaspect of the present invention. This memory includes a write controller311, a write pulse generator 312, a cell array 313, a pulse counter 314,a switch 315, a stop signal generator 316, and a high voltage detector317.

[0198] The write controller 311 receives an external control signal anda write stop signal WS, to control the write pulse generator 312 towrite data into cell transistors of the cell array 313. The write pulsegenerator 312 provides a write pulse to the cell array 313 and to thepulse counter 314, which counts the number of write pulses, i.e., thenumber of verify operations. To easily change a count in the pulsecounter 314, direct input wiring to a NAND circuit and output wiring ofan inverter may be fabricated in advance, to selectively connect theNAND circuit to the wiring during manufacturing.

[0199] The switch 315 is connected to the standard number “N” of pulseapplications or to the delivery test maximum “n” (n<N) of pulseapplications. According to the number N or n, the stop signal generator316 provides the write controller 311 with a write stop signal WS. Theswitching of the switch 315 is controlled in response to a switchcontrol signal SC provided by the high voltage detector 317 depending onwhether or not an external high voltage is applied thereto.

[0200]FIG. 31 shows an essential part of the semiconductor memory deviceof FIG. 30, and FIG. 32 explains the operation of the circuit of FIG.31. In FIGS. 31 and 32, a reference mark QCOi is a pulse count signal ofeach stage.

[0201] A stop signal WS(N) corresponding to the standard maximum ofpulse applications is prepared according to the pulse count signalsQCO2, QCO3, and QCO4. A stop signal WS(n) corresponding to the deliverytest maximum of pulse applications is prepared according to the pulsecount signals QCO0, QCO1, and QCO2. When switch control signal SC fromthe high voltage detector (EWCMGN) 317 is at low level L, i.e., when nohigh voltage is applied, the stop signal WS(N) corresponding to thestandard number N is provided with the 21st pulse of the pulse signalQCO0. When the signal SC from the detector 317 is at high level H, i.e.,when a high voltage is applied to a given terminal, the stop signalWS(n) corresponding to the delivery test maximum n is provided with the4th pulse of the pulse signal QCO0.

[0202] In the above explanation, the maximum number of write pulsesapplied until the stop signal WS is provided is changed between thestandard number guaranteed for a user and the number for a deliverytest. Instead of changing the pulse application numbers, the width of awrite pulse may be changed. For example, the width of a write pulse fora delivery test may be shorter than that of a write pulse for standarduse, to impose severer conditions on the delivery test. The width of awrite pulse for the delivery test may be narrowed once a high voltage isapplied to a given terminal. Although the above explanation is relatedto a write operation, the same is applicable for an erase operation.

[0203] In this way, a semiconductor memory device according to the thirdaspect of the present invention incorporates an internal algorithm thatis capable of changing the number of write or erase operations of memorycells. Semiconductor memory devices with this arrangement will rarelyfail on the user side once passing a delivery test.

[0204] A semiconductor memory device according to a fourth aspect of thepresent invention will be explained with reference to FIGS. 33 to 36.

[0205]FIG. 33 shows a memory cell (MCo) of the semiconductor memorydevice (flash memory) according to the fourth aspect of the presentinvention. The memory cell (cell transistor) has a floating gate FG. Thefloating gate is disposed between a source and a drain and is insulatedfrom other regions. A control gate CG is formed above the floating gateFG.

[0206] To read data from the cell transistor, a gate voltage Vg isequalized with a power source voltage Vcc, a drain voltage Vd to aboutone volt, and a source voltage Vs to a ground level Vss. The data isdetermined to be “1” or “0” depending on whether or not a drain currentflows.

[0207] To write data to the cell transistor, the drain voltage Vdapplied to a drain DD is set to a high voltage (usually Vcc<highvoltage<Vpp), the gate voltage Vg applied to the control gate CG to thewrite voltage Vpp (up to about +10 volts), and the source voltage Vsapplied to a source SS to the ground level Vss. Charges are injectedfrom the drain DD to the floating gate FG, to write data of “0.” Thewrite voltage is becoming lower in these days. Accordingly, it isrequired to effectively apply such low write voltage to the drain.

[0208]FIG. 34 shows a conventional flash memory related to the fourthaspect of the present invention. The flash memory includes a row addressbuffer 411, a row decoder 412, a column address buffer 413, a columndecoder 414, a buffer 415, a write voltage supply transistor 416, asense amplifier 417, and a bus 418. The flash memory also includes bitlines BLs, word lines WLs, write data /WD (inverted level), and a writecontrol signal W.

[0209] To read data from a memory cell MCo, a word line WL and a bitline xL are selected according to a row address and a column address,respectively. The sense amplifier 417 checks to see whether or not acurrent flows through the selected memory cell (cell transistor) MCo, todetermine whether data stored in the memory cell is “1” or “0.”

[0210] To write data to a memory cell MCo, the write control signal Wsets word and bit line selection signals to a write voltage Vpp. Whenwrite data /WD is provided, the transistor 416 is turned ON so that thewrite voltage Vpp (decreased by the threshold voltage of the transistor416) is applied to the bus 418 and to the drain of the memory cell. Theflash memory of FIG. 34 employs a sufficiently high voltage as the writevoltage Vpp, so that the write voltage supply transistor 416 may be ann-channel type MOS transistor. In this case, the write voltage Vpp isdecreased by the threshold voltage of the transistor and is applied tothe drain of the cell transistor MCo.

[0211] Recent flash memories employ a single power source of, forexample, five volts, which requires a reduced write voltage. With such afive-volt single power source, the threshold voltage of the writevoltage supply transistor 416 may decrease a supply voltage lower thanthe write voltage Vpp, to supply an insufficient write voltage to thedrain of a memory cell.

[0212] An object of the fourth aspect of the present invention is toprovide a semiconductor memory device that is capable of supplying awrite drain voltage with no influence of the threshold voltage of awrite voltage supply transistor, thereby correctly writing data to amemory cell even with a low write voltage.

[0213]FIG. 35 shows a semiconductor memory device according to anembodiment of the fourth aspect of the present invention. Unlike theconventional semiconductor memory device of FIG. 34, the semiconductormemory device of FIG. 35 employs a p-channel type MOS transistor 406 asa write voltage supply transistor, and a buffer 405 receives a positivelogic signal WD as write data. A row address buffer 401, a row decoder402, a column address buffer 403, a column decoder 404, and the buffer405 of FIG. 35 correspond to the row address buffer 411, row decoder412, column address buffer 413, column decoder 414, and buffer 415 ofthe related art of FIG. 34, respectively. The semiconductor memorydevice of FIG. 35 involves bit lines BLs, word lines WLs, and a writecontrol signal W.

[0214] To read data from a memory cell MCo of the semiconductor memorydevice of FIG. 35, a word line WL and a bit line BL are selectedaccording to a row address and a column address, respectively. A senseamplifier 407 checks to see whether or not a current flows through theselected memory cell (cell transistor) MCo, to determine whether thememory cell stores data “1” or “0.”

[0215] To write data to a memory cell, the write control signal W setsword and bit line selection signals to a write voltage Vpp. The buffer405 converts write data WD into a signal having the write voltage Vpp.When the gate signal to the write voltage supply transistor 406 becomeslow level L, the transistor 406 is turned ON to supply the write voltageVpp to a bus 408. Since the transistor 406 is a p-channel type MOStransistor, the write voltage Vpp applied to the source of thetransistor 406 is not dropped by the threshold voltage of the transistor406. As a result, the potential of the bus 408 is increased to nearlythe write voltage Vpp, thereby effectively applying the write voltageVpp to the drain of the selected cell transistor MCo. Even when thisflash memory is driven with a single five-volt power source, the lowwrite voltage Vpp is sufficient to write data to memory cells.

[0216]FIG. 36 is a circuit diagram showing an essential part of asemiconductor memory device according to another embodiment of thefourth aspect of the present invention. The part shown in FIG. 36corresponds to the voltage supply transistor 416 and buffer 415 of theconventional semiconductor memory device of FIG. 34.

[0217] The embodiment of FIG. 36 employs an n-channel type MOStransistor as a write voltage supply transistor 426, similar to theprior art of FIG. 34. Instead of the buffer 415 of FIG. 34, however, theembodiment of FIG. 36 employs n-channel type MOS transistors 4251, 4252,and 4253, inverters 4255, 4256, and 4257, and a bootstrap circuit 4253made of a capacitor. When write data WD changes from high level H to lowlevel L, the bootstrap circuit 4253 sets the gate of the transistor 4251to high level H. Then, the gate of the transistor 4252 changes to lowlevel L, to increase the potential of a node N40. At this time, thecapacitor 4253 further increases the level of the gate of the transistor4251 up to about Vpp+Vcc. This potential is applied to the gate of thewrite voltage supply transistor 426, to increase the potential of a bus418 to about the write voltage Vpp.

[0218] In this way, the gate of the write voltage supply transistor 426made of an n-channel type MOS transistor receives the data signalincreased to the write voltage Vpp or over, and the bus 418 receives avoltage equal to the write voltage Vpp, so that data is effectivelywritten to a selected memory cell with the low write voltage Vpp.

[0219] As explained above, the semiconductor memory device according tothe fourth aspect of the present invention provides a sufficient writedrain voltage even under the influence of the threshold voltage of awrite voltage supply transistor, to correctly write data to a selectedmemory cell even with a low write voltage.

[0220] A semiconductor memory device according to a fifth aspect of thepresent invention will be explained with reference to FIGS. 37 to 45.

[0221]FIG. 37 shows a conventional semiconductor memory device (flashmemory) related to the fifth aspect of the present invention. Thesemiconductor memory device of FIG. 37 includes a row decoder 512, acolumn decoder 514, a sense amplifier 517, and a source voltage supplycircuit 519. The semiconductor memory device includes memory cells (celltransistor) MCs each made of an n-channel type MOS (MIS) transistor,word lines WLs, bit lines BLs, and source lines SLs. The source voltagesupply circuit 519 is connected to the sources of the memory cells MCsin a memory cell array through the source lines SLs, to electricallycollectively erase the memory cells. Each memory cell MC is the same asthat shown in FIG. 33.

[0222]FIG. 38 shows an example of the row decoder 512, FIG. 39 shows anexample of the column decoder 514, and FIG. 40 shows a bit line transfergate 5145 of the column decoder 514 of FIG. 39.

[0223] In FIG. 38, the row decoder 512 has a power source 5121, gatesRG1 to RGn for receiving a row address, a transistor 5122 connectedbetween the gates RG1 to RGn and the power source 5121, and an inverter(transistors 5123 and 5124) connected between the power source 5121 anda low voltage supply source Vss (at a ground level GND of zero volts).The inverter controls the level of a word line WL. For example, aselected word line WL corresponding to a row address with all high level(H) bits to turn ON the gates RG1 to RGn receives the output Vcc of thepower source 5121 through the transistor 5123, and every otherunselected word line WL receives the low voltage Vss of zero voltsthrough the transistor 5124.

[0224] In FIG. 39, the column decoder 514 has a power source 5141, gatesCG1 to CGm for receiving a column address, a transistor 5142 between thegates CG1 to CGm and the power source 5141, a bit line transfer gate5145, and an inverter (transistors 5143 and 5144) between the powersource 5121 and the low voltage Vss. The inverter controls the bit linetransfer gate 5145. When input column address bits are each of highlevel H to turn ON the gates CG1 to CGm, a selected bit line BL isconnected to the sense amplifier 517.

[0225] In FIG. 40, bit line transfer gates 5145 l to 5145 m areconnected to the sense amplifier 517 through a bus BUS. Only a bit lineselected through the bit line transfer gates 5145 l to 5145 m isconnected to the sense amplifier 517. Accordingly, the contents of amemory cell MC located at an intersection of the selected word and bitlines are provided outside through the sense amplifier 517.

[0226] Generally, all cell transistors (memory cells) MCs of a flashmemory are simultaneously erased for the sake of simplicity of circuitdesign. The erasing is repeated until all the cell transistors arecompletely erased. Due to statistical reasons, some cell transistors ina flash memory are relatively easily erased and some are not. If acharacteristic difference between these two kinds of memory cells isvery large, the memory cells that are easily erased may be overerased.The characteristics of cell transistors easily deviate from one anotherduring a wafer processing and through the repetition of write and eraseoperations.

[0227] To write or erase data to or from cell transistors of a flashmemory, the floating gates of the cell transistors are charged ordischarged. If a memory cell is overerased, the floating gate thereofholds charges whose polarity is opposite to a polarity formed when datais written thereto. Namely, when a memory cell has an oppositely chargedfloating gate, it is determined that the cell has been overerased.

[0228] Nonvolatile semiconductor memories (flash memories) usually havea NOR-type cell array. According to this type, drains of celltransistors (n-channel type MOS transistors) are connected to oneanother bit line by bit line. When selecting a cell transistor (memorycell), the commonly connected cell transistors are uniformly biased, andthen, a positive bias voltage is applied only to the gate of the celltransistor to be selected and zero volts is applied to the gates ofunselected cell transistors.

[0229] The sources of all cell transistors are grounded through thesource voltage supply circuit 519 in FIG. 37. Since the cell transistorsare enhancement MOS (MIS) transistors (n-channel type MOS transistors),the unselected cell transistors do not pass current, and only theselected cell transistor passes or does not pass a current depending onthe quantity of charges accumulated in the floating gate thereof.According to the current flowing through the selected cell transistor,data “0” or “1” is determined.

[0230] When an enhancement cell transistor of the flash memory isovererased, the characteristics of the overerased transistor change tothose of a depletion transistor. To solve this problem, the NOR-typeflash memory supplies no current to unselected cell transistors when nobias is applied to the gates of the transistors. The overerased celltransistor, however, shows depletion characteristics, and therefore,passes a current even if it is unselected. Namely, even if a selectedcell transistor passes no current, the overerased unselected celltransistor may pass a current, to erroneously determine data “0” as data“1,” or data “1” as data “0.”

[0231] An object of the fifth aspect of the present invention is toprovide a semiconductor memory device that correctly reads data even ifthere is an overerased cell transistor.

[0232]FIG. 41 shows a flash memory according to an embodiment of thefifth aspect of the present invention. The flash memory has a rowdecoder 502, a column decoder 504, a sense amplifier 507, and a sourcevoltage supply circuit 509. The arrangement of FIG. 41 is basically thesame as that of FIG. 37 and is characterized by the row decoder 502.

[0233]FIG. 42 shows an example of the row decoder 502 of FIG. 41. Therow decoder 502 has a positive power source 5021 for generating a givenpositive voltage, gates RG1 to RGn for receiving a row address, atransistor 5022 between the gates RG1 to RGn and the positive powersource 5021, a negative power source 5025, and an inverter (transistors5023 and 5024) between the positive power source 5021 and the negativepower source 5025. The inverter controls the level of a word line WL.When an input row address has bits each of high level H, the gates RG1to RGn are turned ON, and a corresponding word line (selected word line)WL receives an output Vcc of the positive power source 5021 through thetransistor 5123. At this time, other unselected word lines WLs receiveeach an output (a negative voltage) of the negative power source 5025through the transistor 5024. The negative power source 5025 may be thenegative voltage generator 118 of the semiconductor memory device of therelated art of FIG. 5.

[0234] When the output of the negative power source 5025 is applied tothe gate of an overerased cell transistor MC showing depletioncharacteristics, the voltage causes the transistor to be unselected topass no current. Namely, the negative output voltage of the negativepower source 5025 suppresses the gate voltage of the overeraseddepletion n-channel type MOS transistor below a threshold voltage.Consequently, even if a selected bit line involves overerased celltransistors, the contents of a cell transistor selected by a word lineare correctly read out through the sense amplifier 507.

[0235]FIG. 43 shows a semiconductor memory device according to anotherembodiment of the fifth aspect of the present invention. Thissemiconductor memory device has first and second row decoders 5221 and5222 corresponding to the row decoder 502 and source voltage supplycircuit 509 of FIG. 41. The second row decoder 5222 applies a voltageequal to or greater than the level of a selected bit line to sourcelines SWLs corresponding to unselected word lines WLs. A column decoder524, a sense amplifier 527, etc., of FIG. 43 are the same as those ofFIG. 41.

[0236] When reading data from a memory cell MC, the first row decoder5221 applies a normal voltage Vcc to a selected word line WL, to selectmemory cells (cell transistors) connected to the selected word line WL.The second row decoder 5222 applies a low power source voltage Vss ofzero volts to the source line SWL of the cell transistors connected tothe selected word line, and a voltage equal to or greater than the drainvoltage of the selected bit line to the source lines of cell transistorsconnected to unselected word lines. Overerased cell transistors amongthe unselected transistors can be disconnected because the gate voltagethereof becomes lower than the source voltage thereof. A voltage appliedto the sources of the memory cells connected to the unselected wordlines in the read operation may be equal to the level of the selectedbit line. Even if a channel is produced due to the overerasing, nocurrent flows if there is no potential difference between the drain andsource of each overerased cell. Accordingly, the overerased unselectedcell transistors never influence the read operation.

[0237]FIG. 44 shows examples of the first and second row decoders 5221and 5222 of FIG. 43, and FIG. 45 shows a part of the second row decoder5222.

[0238] In FIG. 44, the first row decoder 5221 has a power source (Vcc)52211, a NAND gate 52212 for receiving a row address, and an inverter52213. The second row decoder 5222 has a power source (Vcc) 52221, aNAND gate 52222 for receiving the row address, inverters 52223 and52224, and a power source 52225. The power source 52225 supplies avoltage higher than the drain voltage of a selected bit line to thesources of cell transistors connected to unselected word lines whenreading data, as shown in FIG. 45.

[0239] As explained above, a flash memory according to the fifth aspectof the present invention correctly reads data of “0” or “1” even if somememory cells have been overerased during a wafer process or throughrepetitive write and read operations. The fifth aspect of the presentinvention thus improves the yield and reliability of flash memories.

[0240] A semiconductor memory device according to a sixth aspect of thepresent invention will be explained with reference to FIGS. 46 to 49.Similar to the fifth aspect, the sixth aspect correctly reads data frommemory cells of a flash memory even if some memory cells are overerasedduring a collective erase operation.

[0241] As explained before with reference to FIGS. 37 to 40, celltransistors (memory cells) of a flash memory are simultaneously erasedfor the sake of simplicity of circuit designing. The erasing is repeateduntil all the cell transistors are completely erased. Some celltransistors of the flash memory are relatively easily erased and someare not. When the cells are collectively erased, the cells that areeasily erased may be overerased. The characteristics of the celltransistors easily differ from one another during a wafer process andthrough the repetition of write and erase operations. Accordingly, theovererasing of cells frequently occurs.

[0242] Generally, flash memories have a NOR-type cell array. Accordingto this type, drains of n-channel type MOS transistors (celltransistors) are connected to one another bit line by bit line. Whenselecting a cell transistor, the commonly connected cell transistors areuniformly biased, and then, a positive bias voltage is applied to onlythe gate of the cell transistor to be selected and zero volts is appliedto the gates of unselected cell transistors. Since the cell transistorsare enhancement n-channel type MOS transistors, the unselected celltransistors do not pass a current, and only the selected cell transistorpasses or does not pass a current depending on the quantity of chargesstored in the floating gate thereof. According to the current flowingthrough the selected cell transistor, data “0” or “1” is determined.

[0243] When an enhancement cell transistor of the flash memory isovererased, the characteristics of the overerased transistor change tothose of a depletion transistor. To solve this problem, the NOR-typeflash memory supplies no current to unselected cell transistors when nobias is applied to the gates of the transistors. The overerased celltransistor, however, shows depletion characteristics, and therefore,passes current even if it is unselected. Namely, even if a selected celltransistor passes no current, the overerased unselected cell transistorpasses current, to erroneously determine data “0” as data “1,” or data“1” as data “0.”

[0244] An object of the sixth aspect of the present invention is toprovide a semiconductor memory device that correctly reads data bysaving overerased cell transistors.

[0245]FIG. 46 shows an essential part of the semiconductor memory deviceaccording to the sixth aspect of the present invention. Thesemiconductor memory device includes a row decoder 602, a column decoder604, a sense amplifier 607, memory cells (cell transistors) MCs eachmade of an n-channel type MOS transistor, word lines WLs, and bit linesBLs. Each memory cell MC is the same as that shown in FIG. 33. Note, thesense amplifier 607 receives a signal OECK which is changed to highlevel H when an erase verifying operation is carried out (with referenceto Step 613 in FIG. 49).

[0246]FIG. 47 shows an example of the sense amplifier 607 of FIG. 46.The sense amplifier 607 includes n-channel type MOS transistors 6071,6072, 6073, 6074, 6075, and 6077, and p-channel type MOS transistors6076 and 6078. Note, as sgown in FIG. 47, the signal OECK, which isapplied to the sense amplifier 607, is also applied to the gate of theMOS transistors 6077.

[0247] To detect overerased cell transistors after a collective eraseoperation, the row decoder 602 sets all word lines WL1, WL2, . . . ofthe collectively erased cell array to low level L. Thereafter, thecolumn decoder 604 successively selects column gates G601, G602, . . . ,to successively connect the bit lines BL1, BL2, . . . to the senseamplifier 607. Whenever a bit line involving an overerased celltransistor is connected to the sense amplifier 607, the sense amplifier607 provides an output of low level L. Then, the transistor 6077 of thesense amplifier 607 is turned ON to increase a drive current of thesense amplifier 607, thereby preventing a current from flowing throughthe overerased cell. At the same time, the row decoder 602 scans theword lines WL1, WL2, . . . , to detect the overerased cell that hascaused the sense amplifier 607 to provide an output of high level H.

[0248]FIG. 48 schematically shows a system employing the semiconductormemory device according to the sixth aspect of the present invention.The system includes the flash memory 610, a read only memory (ROM) 620,and a central processing unit (CPU) 630.

[0249] The ROM 620 stores an algorithm shown in FIG. 49, and the CPU 630controls the flash memory 610 according to the algorithm. Namely, theCPU 630 saves overerased cells of the flash memory 610 according to thealgorithm stored in the ROM 620.

[0250]FIG. 49 shows a flow of the algorithm stored in the ROM 620 ofFIG. 48. After the start of an erase operation of the flash memory, stepS611 carries out a write-before-erase operation. Namely, beforecollectively erasing the memory cell array of the flash memory, data “0”is written to every cell transistor of the memory cell array.

[0251] Step S612 carries out the collective erase operation, and stepS613 carries out an erase verify operation. Note, in the step S613, asignal OECK, which is applied to the sense amplifier 607 and the gate ofthe MOS transistor 6077, is changed from low level L to high level H,when the erase verify operation is carried out. Namely, electrons aregradually discharged from the floating gates of all cell transistors ofthe memory cell array, to achieve the erase operation. Step S614 checksto see whether or not there is an overerased cell. If there is noovererased cell, the erase process ends. If there is an overerased cell,step S615 is carried out. As explained with reference to FIGS. 46 and47, step S614 detects overerased cell transistors one by one.

[0252] Step S615 writes data to the overerased cell (overerased bit)detected in step S614. Step S616 carries out the same test as step S614to check for overerasing. If the overerased cell detected in step S614restores a normal erased state due to the write operation of step S615,and if there is no other overerased cell, step S617 carries out the sameerase verify operation as step S613. If the erase verify operation ofstep S617 is successful, the erase process ends. If the erase verifyoperation of step S617 fails, steps S618 and S619 carry out the eraseand erase verify operations.

[0253] If the overerased cell detected in step S614 restores a normalerased state due to the write operation of step S615 and if anotherovererased cell is found in step S616, step S615 writes data to theovererased cell newly found. In this way, data is written to everyovererased cell of the memory cell array, to restore a normal erasedstate in every overerased cell.

[0254] The erase process of FIG. 49 is stored in the ROM 620 of thesystem of FIG. 48, and the CPU 630 reads and executes the erase process.Alternatively, the erase process of FIG. 49 may be realized by hardware.For example, the flash memory itself may incorporate a logic circuitthat achieves the erase process.

[0255] As explained above, the sixth aspect of the present inventionsaves overerased cell transistors and correctly reads data. Even ifmemory cells of a semiconductor memory device are overerased during awafer process or through the repetition of write and erase operations,the sixth aspect of the present invention correctly reads data out ofthe semiconductor memory device, thereby improving the yield andreliability of the semiconductor memory device.

[0256] A semiconductor memory device according to a seventh aspect ofthe present invention will be explained with reference to FIGS. 50 to61.

[0257]FIG. 50 shows a memory cell (MC) of an electrically collectivelyerasable nonvolatile semiconductor memory device (a flash memory)according to the seventh aspect of the present invention. The memorycell (cell transistor) has a floating gate FG. The floating gate isdisposed between a source and a drain and is insulated from otherregions. A control gate CG is formed above the floating gate FG.

[0258] To write data to the cell transistor, a drain voltage Vd appliedto a drain DD is set to, for example, 6 V, a gate voltage Vg applied tothe control gate CG to a write voltage (erase voltage) Vpp, and a sourcevoltage Vs applied to a source SS to zero. Charges are injected from thedrain DD to the floating gate FG, to write data of “0.”

[0259] To erase data from the cell transistor, the gate voltage Vg anddrain voltage Vd are set to a floating state, and the source voltage Vsis set to the erase voltage Vpp. Electrons are extracted from thefloating gate FG to the source SS, to erase the cell transistor, i.e.,to write data “1” to the cell transistor. To read data from the celltransistor, the gate voltage Vg is set to a power source voltage Vcc,the drain voltage Vd to about one volt, and the source voltage Vs tozero. It is tested to see whether or not a drain current flows, todetermine whether the written data is “1” or “0.”

[0260]FIG. 51 shows a conventional semiconductor memory device relatedto the seventh aspect of the present invention. The semiconductor memorydevice includes a block address buffer 710, block selecting gates 7101and 7102, a row address buffer 711, a row decoder 712, a column addressbuffer 713, a column decoder 714, a data I/O buffer 715, a write circuit716, a sense amplifier 717, and source voltage supply circuits 7191 and7192. The semiconductor memory device also includes bit lines BLs, wordlines WLs, memory cells MCs, a write control signal W that becomes highlevel H when writing data, and an erase signal E that becomes high levelH when erasing data.

[0261] This semiconductor memory device operates similarly to thesemiconductor memory device of FIG. 2. The semiconductor memory deviceof FIG. 51 additionally has the block address buffer 710 and blockselecting gates 7101 and 7102 corresponding to blocks B1 and B2 ofmemory cells. One of the block selecting gates 7101 and 7102 is selectedaccording to a block selection signal from the block address buffer 710,to connect one of the blocks B1 and B2 to the write circuit 716 or thesense amplifier 717. In each of the blocks B1 and B2 of memory cells,sources of the memory cells are connected to one another. With the useof the source voltage supply circuits 7191 and 7192 provided for theblocks B1 and B2, respectively, the blocks are separately erasable.

[0262] To erase the cells of one of the blocks B1 and B2, the erasesignal E of high level H is supplied to the row address buffer 711 andcolumn address buffer 713, to set the outputs of the buffers 711 and 713to an unselected logic (for example, each complementary output being atlow level L). This results in putting all word and bit lines WLs and BLsin an unselected state. The erase signal E and a block selection signalfrom the block address buffer 710 are supplied to the source voltagesupply circuits 7191 and 7192. One of the source voltage supply circuitsselected with the block selection signal of, for example, high level H,is used to supply an erase voltage Vpp to the selected block.Consequently, all cells in the selected block are erased.

[0263] When writing data, the write control signal W of high level H isprovided to the row address buffer 711 and column address buffer 713.Accordingly, a selected word line WL is set to the write level Vpp, anda selected bit line BL is connected to the write circuit 716 through oneof the block selection gates 7101 and 7102 selected according to theblock selection signal. The write circuit 716 provides the selected bitline BL of the selected block with a write voltage of, for example, 6volts to write data.

[0264] The conventional semiconductor memory device of FIG. 51 may erasea block of memory cells but is unable to simultaneously erase aplurality of blocks of memory cells. Namely, the semiconductor memorydevice of FIG. 51 sequentially erases blocks one after another, andthereafter, verifies the erased blocks. Accordingly, this semiconductormemory device requires a long time for erasing a plurality of blocks andmust carry out a complicated verify process.

[0265] An object of the seventh aspect of the present invention is toprovide a semiconductor memory device capable of simultaneously erasinga plurality of blocks of memory cells and easily verifying the erasedblocks.

[0266]FIG. 52 shows a semiconductor memory device according to anembodiment of the seventh aspect of the present invention. Thissemiconductor memory device includes a block address buffer 701,expected value storage circuits 7021 and 7022, coincidence circuits 7031and 7032, a logic circuit (NAND gate) 704, a row address buffer 721, arow decoder 722, a column address buffer 723, a column decoder 724, adata I/O buffer (multiplexer) 725, write circuits 7261 and 7262, senseamplifiers 7271 and 7272, and source voltage supply circuits 7091 and7092. The semiconductor memory device also includes bit lines BLs, wordlines WLs, memory cells MCs, a write control signal W that becomes highlevel H when writing data, and an erase control signal E that becomeshigh level H when erasing data. Compared with the conventionalsemiconductor memory device of FIG. 51, the embodiment of FIG. 52additionally has the expected value storage circuits 7021 and 7022,coincidence circuits 7031 and 7032, multiplexer (data I/O buffer) 725,and NAND gate 704.

[0267] To erase blocks of memory cells, the source voltage supplycircuits 7091 and 7092 latch a selection signal of the block addressbuffer 701 when a latch control signal LT becomes high level H. Theerase control signal E is set to high level H to activate all the sourcevoltage supply circuits that have latched the selection signal, therebysimultaneously erasing blocks of memory cells.

[0268] FIGS. 53 to 55 show examples of the source voltage supply circuit7091 (7092), expected value storage circuit 7021 (7022), and coincidencecircuit (7031 (7032) of the semiconductor memory device of FIG. 52,respectively.

[0269] In FIG. 53, the source voltage supply circuit 7091 (7092) has aNAND gate 731 for receiving the block address signal (block selectionsignal) and latch control signal LT, a latch circuit formed of a NANDgate 732 and an inverter 733, a NAND gate 734 for receiving an output ofthe latch circuit as well as the erase control signal E, p-channel typeMOS transistors 736 and 737 connected to an erase power source Vpp, andan n-channel type MOS transistor 738. In FIG. 54, the expected valuestorage circuit 7021 (7022) includes inverters 741, 744, 745, 746, and750, and NAND gates 742, 743, 747, 748, and 749. The NAND gate 743 andinverter 744 form a latch circuit. An output of the latch circuit iscontrolled in response to an inversion control signal INV. In FIG. 55,the coincidence circuit 7031 (7032) has an inverter 753, NAND gates 751,752, and 755, and an exclusive OR gate 754. The NAND gate 752 andinverter 753 form a latch circuit, which determines whether or not anoutput of the sense amplifier 7272 (7272) agrees with a reference outputof the expected value storage circuit 7021 (7022).

[0270] In the above arrangements, an erase operation is carried out bylatching the block address signal. In this case, only the blocks thathave latched the block address signal are erased with the erase controlsignal E. To carry out a write-before-erase operation, the latch controlsignal LT is set to high level H, and write data “0” is latched by theexpected value storage circuit 7021 (7022). Namely, the data I/O buffer725 transfers data “0” to the expected value storage circuit 7021 (7022)selected by the block address signal, and the circuit 7021 (7022)latches the data “0” when the latch control signal LT becomes high levelH. An output of the coincidence circuit 7031 (7032) of an unselectedblock is forcibly set to high level H. When the write control signal Wbecomes high level H, the data is written to the selected block(s).

[0271] To carry out a verify operation, the coincidence circuit 7031(7032) compares the expected value stored in the expected value storagecircuit 7021 (7022) with an output of the sense amplifier 7271 (7272).Outputs of the coincidence circuits 7031 and 7032 are sent to the NANDgate 704. If the data has been sufficiently written, the sense amplifier7271 (7272) provides a low level output, and therefore, the coincidencecircuit 7031 (7032) provides a high level output. If the data has beensufficiently written to all the selected blocks, the NAND gate 704provides a verify output VER of low level L to confirm that the data hasbeen successfully written to a given address of each selected block. Tocarry out an erase verify operation, the expected value is invertedaccording to the inversion control signal INV and is verified. If dataat a given address is “1” in every selected block, the verify output VERwill be low level similar to the write operation, to confirm that datahas been erased. In this way, according to the semiconductor memorydevice of this embodiment, the output of the NAND gate 704 becomes lowlevel L only when all outputs of the coincidence circuits 7031 and 7032are each at high level H. If any one of the coincidence circuitsprovides a low level output, it is confirmed that there is a defectivebit.

[0272]FIG. 56 is a circuit diagram showing a semiconductor memory deviceaccording to another embodiment of the seventh aspect of the presentinvention. This embodiment employs an expected value generator 7041(7042) instead of the expected value storage circuit 7021 (7022) of FIG.52.

[0273] The semiconductor memory device of FIG. 52 may be realizedwithout a unit for storing random data because reference data necessaryfor the write-before-erase and erase operations are “0” and “1” for allbits. According to the embodiment of FIG. 56, the expected valuegenerator 7041 (7042) latches its block address signal (selectionsignal) when the latch control signal LT becomes high level H. Theexpected value generator 7041 (7042) that has latched the selectionsignal forcibly provides data of “0.” When carrying out an erase verifyoperation, the expected value is inverted in response to the inversioncontrol signal INV.

[0274]FIG. 57 shows an example of the expected value generator 7041(7042) of FIG. 56. The expected value generator has inverters 763, 764,767, and 768, NAND gates 761, 762, 766, 769, and 770, and a NOR gate765. The NAND gate 762 and inverter 763 form a latch circuit. Thisexpected value generator forcibly sets reference data to low level Lwhen the latch circuit (762, 763) latches an address data signal (blockselection signal), and when the inversion control signal INV becomeshigh level H, changes the reference data to high level H.

[0275]FIG. 58 is a circuit diagram showing a semiconductor memory deviceaccording to another embodiment of the seventh aspect of the presentinvention. This embodiment employs a block selection signal storagecircuit 7051 (7052), a write circuit 7161 (7162), and a data inversioncircuit 7061 (7062), in place of the expected value storage circuit 7021(7022), write circuit 7261 (7262), and coincidence circuit 7031 (7032)of FIG. 52. Namely, this embodiment controls the write-before-eraseoperation, verify operation thereof, and erase verify operation inresponse to a selection signal stored in the storage circuit 7051(7052).

[0276] The semiconductor memory device of FIG. 58 carries out thewrite-before-erase operation by writing data to cells in a blockspecified by a block selection signal (a block address signal). The datainversion circuit 7061 (7062) inverts data of the sense amplifier 7271(7272) between the erase verify operation and the write verifyoperation, and provides an output of high level H if the write or eraseoperation has been sufficiently done. At this time, the sense amplifier7271 (7272) of an unselected block always provides an output of highlevel H due to an output signal of the storage circuit 7051 (7052).Consequently, the write-before-erase operation and erase operation areachieved as explained before.

[0277] FIGS. 59 to 61 show examples of the block selection signalstorage circuit 7051 (7052), write circuit 7161 (7162), and datainversion circuit 7061 (7062) of the semiconductor memory device of FIG.58, respectively.

[0278] In FIG. 59, the block selection signal storage circuit 7051(7052) has a NAND gate 771 for receiving the block address signal (blockselection signal) and latch control signal LT, and a latch circuitformed of a NAND gate 772 and an inverter 773. In FIG. 60, the writecircuit 7161 (7162) has an inverter 781, a NOR gate 782, a NAND gate783, p-channel type MOS transistors 785 and 786 for receiving a writepower source Vpp, and an n-channel type MOS transistor 787. In FIG. 61,the data inversion circuit 7061 (7062) has inverters 792 and 793, andNAND gates 791, 794, 795, and 796. The data inversion circuit inverts anoutput of the sense amplifier 7271 (7272) in response to the inversionsignal INV and supplies the inverted signal to the NAND gate 704.

[0279] The block selection signal storage circuit 7051 (7052) of FIG. 59latches the block address signal in the latch circuit (772, 773) inresponse to the latch control signal LT of high level H. In the blockthat has latched the block selection signal, the write voltage Vpp isapplied to a bus when the write control signal W is set to high level H,irrespective of the level of input data.

[0280] The data inversion circuit 7061 (7062) of FIG. 61 inverts theoutput of the sense amplifier 7271 (7272) to low level L (a writtenstate) when the inversion control signal INV becomes low level L in thewrite verify operation, and provides an output of high level H to theNAND gate 704. In the erase verify operation, the inversion controlsignal INV is set to high level H. In an unselected block, the blockselection signal (block address signal) is at low level L, andtherefore, the output to the NAND gate 704 is forcibly set to high levelH. As a result, optional blocks are simultaneously erased. Themultiplexer 725 follows the block selection address to supply write datato given blocks and data from given sense amplifiers to the outside.

[0281] As explained above, the semiconductor memory device according tothe seventh aspect of the present invention employs a latch circuit of ablock selection signal in a source voltage supply circuit of each cellblock. The source voltage supply circuits are simultaneously activated.Each cell block is provided with a sense amplifier, an expected valuegenerator, and a circuit for confirming the coincidence of a senseamplifier output and an expected value. The semiconductor memory devicefurther has a circuit for providing an AND output of the coincidencecircuits, to simultaneously carry out erase and verify operations.

[0282] A semiconductor memory device according to an eighth aspect ofthe present invention will be explained with reference to FIGS. 62 to73.

[0283] Semiconductor memories such as EEPROMs (electrically erasableprogrammable read only memory) have attracted attention recent years. Inparticular, flash memories have attracted considerable attention as areplacement for magnetic storage devices because they are suitable forintegration. Although the following explanation of the present inventionis made in connection with the flash memories, the present invention isalso applicable for standard EEPROMs.

[0284] Each memory cell of a flash memory has a floating gate and acontrol gate. Data is stored in the cell depending on whether or not thefloating gate holds charges. To write data to the cell, a high voltageof about +12 V is applied to the control gate, a voltage of about +6 Vis applied to a drain, a source is grounded, and hot electrons areinjected into the floating gate. To read data from the cell, a voltageof about +5 V is applied to the control gate, a voltage of about +1 V isapplied to the drain, the source is grounded, and charges accumulated inthe floating gate are sensed to detect a current difference. To erasedata from the cell, a high voltage of about +12 V is applied to thesource, the control gate is grounded, and the drain is opened to producea tunnel current between bands. As a result, electrons are drawn fromthe floating gate to the source. When writing data to the cell, a draincurrent of about several milliamperes flows, and a current of severaltens of milliamperes is needed in total. Accordingly, a power source fora normal voltage Vcc (+5 V) and a power source for a high voltage Vpp(+12 V) are needed.

[0285]FIG. 62 shows a conventional computer system employing a flashmemory. In addition to a power source for a normal voltage, this systemhas a power source for a high voltage. The system involves a centralprocessing unit (CPU) 821, a ROM/RAM 822, an input/output (I/O) port823, a peripheral device 824, the flash memory 825, the high voltagesupply source 828, and a control port 829 for controlling the highvoltage supply source 828. The high voltage supply source 828 includes ahigh voltage controller 826 and a high voltage generator 827. The highvoltage supply source 828 is controlled by the CPU 821 through thecontrol port 829, to supply and stop a high voltage Vpp to the flashmemory 825. The high voltage generator 827 may be a fixed power sourceor a step-up circuit. In the case of the fixed power source, the highvoltage controller 826 will be simply a switch. In the case of thestep-up circuit, it is activated by the controller 826 only when needed,to reduce power consumption.

[0286] The flash memory 825 is not frequently written to or erased, andtherefore, the high voltage is supplied only when needed. FIG. 63 showssteps of controlling the high voltage supply source 828 when writing anderasing data in the system of FIG. 62. Each of the steps will be brieflyexplained.

[0287] Step S831 provides the control port 829 with an instruction tosupply a high voltage. In the case of the fixed power source, thecontroller 826 is switched to supply the high voltage. In the case ofthe step-up circuit, a step-up operation is started. Step S832 waitsuntil a sufficient high voltage is supplied. In the case of the fixedpower source, this step is omitted.

[0288] Step S833 writes a write command or an erase command to the flashmemory 825. Steps S834 and S835 test a state after the step S833. If noproblem is found, step S836 causes the control port 829 to provide asignal for stopping the high voltage.

[0289] In practice, it is disadvantageous to prepare a high voltagesupply source in addition to a normal voltage supply source. Namely,flash memories operating with a single power source are needed.

[0290] One of such flash memories operating with a single power sourceincorporates a step-up circuit for increasing a normal voltage Vcc to ahigh voltage Vpp. The increased high voltage Vpp is applied to thecontrol gate of a memory cell, and the normal voltage Vcc instead of avoltage of +6 V is applied to the drain thereof. Although this techniquelowers the avalanche breakdown voltage of the drain, it causes littlecurrent to flow to the control gate so that even the internal step-upcircuit having a small current supply capacity will be sufficient toproduce the voltages. When a voltage is applied to the drain, a largequantity of current is needed. In this case, the normal voltage is used.Consequently, the single power source works to write and erase data.

[0291] To reduce the power consumption of the flash memory, it isnecessary to reduce the normal voltage Vcc. When the normal voltage Vcc,which is low, is directly applied to the drain of a memory cell, thenormal voltage Vcc may be lower than the avalanche breakdown voltage. Inthis case, data cannot be written. To solve this problem, a separatehigh voltage source similar to that of the prior art may be used. Manysystems, however, are designed to work with a single power source, sothat, if a flash memory operating with a separate high voltage source isemployed for such systems, software for controlling the high voltagesource must be modified. This modification is troublesome thusdeteriorating the value of the flash memory.

[0292] Since the flash memories do not work with a low voltage, thenormal voltage Vcc must be high. This means that a power sourceproviding a low normal voltage is useless for the flash memories.

[0293] An object of the eighth aspect of the present invention is toprovide a semiconductor memory device employing two power sources thatare easy to use and operable like a single power source.

[0294]FIG. 64 explains the principle of an electrically erasablenonvolatile semiconductor memory device according to the eighth aspectof the present invention.

[0295] This semiconductor memory device (nonvolatile semiconductormemory device) writes or erases data with a high voltage Vpp that ishigher than a normal voltage Vcc used to read data. The nonvolatilesemiconductor memory device has a terminal 806 for receiving the highvoltage Vpp from an external high voltage supply unit 802, in additionto a terminal for receiving the normal voltage. The semiconductor memorydevice has a control terminal 807 for providing the high voltage supplyunit 802 with a control signal to control the supply of the high voltageVpp.

[0296] The signal terminal 807 of the semiconductor memory device(EEPROM) according to the eighth aspect of the present inventionprovides, when needed, the high voltage supply unit 802 with a controlsignal to control the supply of the high voltage. Unlike the prior art,a computer system employing the flash memory of the present invention isnot required to control the high voltage supply unit 802. Similar to thenonvolatile semiconductor memory device having a single power source,the semiconductor memory device of the present invention automaticallycontrols the high voltage supply unit 802 when the semiconductor memorydevice is accessed. Accordingly, the nonvolatile semiconductor memorydevice of the eighth aspect of the present invention is operable in thesame manner as that with a single power source, and requires no softwareamendment.

[0297]FIG. 65 shows steps of providing a high voltage in a systememploying the nonvolatile semiconductor memory device (EEPROM) of thepresent invention.

[0298] Steps S801 to S804, on the left side of FIG. 65, are carried outin the system, and steps S810 to S813 are carried out in the EEPROM.Step S801 sends a write or erase command to the EEPROM. In response tocommand, step S810 determines the kind of the command. If it is acommand to read data, no high voltage is required. In this case, thefollowing steps are skipped, and a read operation is carried out. If thecommand is to write or erase data, step S811 causes the signal terminal807 to send an instruction to start the supply of the high voltage.Then, the process waits until the supplied high voltage reaches asufficient level. If the high voltage power source is a fixed powersource, no wait is required.

[0299] Once the supplied high voltage reaches a sufficient level, stepS812 carries out the write or erase operation. In the case of the eraseoperation, the system is not required to carry out any of the operation,and therefore, the system can commence another operation. In the case ofthe write operation, the system provides write data. In any case, in themiddle of or after the write or erase operation, the system carries outa test in steps S802 and S803.

[0300] Step S804 informs the EEPROM of the completion of the write orerase operation. In response to this, the EEPROM sends a signal to stopthe high voltage in step S813. The write or erase operation usuallyhandles a large quantity of data, so that the write/erase operationcontinues for a certain period. Accordingly, if the EEPROM detects thatthe write/erase operation has ended in the middle of a certain period,it may automatically provide the signal to stop the high voltage.

[0301] In any case, processes to be carried out in the system aresimplified compared with the prior art of FIG. 63.

[0302]FIG. 66 shows a system according to an embodiment of thesemiconductor memory device according to the eighth aspect of thepresent invention.

[0303] This system includes a central processing unit (CPU) 831, aROM/RAM 832, an I/O port 833, a peripheral device 834, a flash memory835 of the present invention, and a DC-DC converter 836 for generating ahigh voltage. The DC-DC converter 836 is turned ON and OFF in responseto a control signal from the flash memory 835.

[0304] In comparison with the system of FIG. 62, the system of FIG. 66has no output port through which the CPU 831 controls the DC-DCconverter 836. The system of FIG. 66 is basically the same as a systemwith a flash memory having a single power source.

[0305]FIG. 67 shows the internal arrangement of the flash memory 835 ofFIG. 66. The flash memory 835 has an address buffer 841, a row decoder842, a column decoder 843, a memory cell matrix 844, a read/writeamplifier 845, an I/O buffer 846, and a controller 847. The flash memory835 involves external I/O terminals such as an address terminal, a dataterminal, a normal power source (Vcc) terminal, a high voltage (Vpp)terminal, and a control terminal. These are the same as those of theconventional flash memory.

[0306] What is different from the conventional flash memory is that theflash memory of FIG. 67 has a command determination unit 849, a voltagetest circuit 850, and an external I/O terminal for controlling a highvoltage supply source. The command determination unit 849 is acomparator for finding, for example, a write or erase command to theflash memory 835. When the CPU 831 provides a write or erase command tothe flash memory 835, the command determination unit 849 detects thecommand and provides the high voltage source control terminal with asignal to start the supply of the high voltage. The commanddetermination unit 849 may be a timer circuit that is reset by a writesignal *WE.

[0307] The voltage test circuit 850 tests if a supplied high voltage isgreater than a predetermined value. FIG. 69 shows an example of thevoltage test circuit 850. This circuit tests not only the high voltagebut also if the high voltage Vpp and normal voltage Vcc applied tointernal power source lines are greater than respective referencevalues. The voltage test circuit 850 includes a sequence circuit 861 forcarrying out a main control operation. A comparator 862 compares avoltage, which comes from an internal power source line 864 and isdivided by a resistor 865, with one of reference voltages r₁ and r₂.These reference voltages are switched from one to another through aswitch 863.

[0308]FIG. 68 shows steps of controlling the external power source forthe flash memory 835. Step S821 checks a signal provided by the CPU 831to the flash memory 835, to see whether or not a high voltage isrequired. If the high voltage is required, step S822 sends an activationsignal to the DC-DC converter 836. The DC-DC converter 836 needs acertain period after the activation thereof until it provides therequired voltage. Accordingly, there is a wait time until the voltagetest circuit 850 confirms in step S823 that the voltage of the internalpower source line is greater than the predetermined value.

[0309] When the voltage becomes greater than the predetermined value,step S824 carries out a write or erase operation. This operationcontinues for a while. After the write or erase operation is complete,step S825 checks to see if data has been correctly written or erased. Ifthe result is YES, step S826 sends a stop signal to the DC-DC converter836. At the same time, the internal power source is connected. The DC-DCconverter 836 then stops supplying the high voltage. Step S827 confirmsthat the connected internal power source is restored to a normalvoltage, and the process ends.

[0310] In FIG. 68, the voltage test circuit 850 of FIG. 67 is used totest the voltage provided by the DC-DC converter 836. It is alsopossible to measure, in advance, a time after the activation of theDC-DC converter 836 until the predetermined voltage is provided, and anoperation that requires the high voltage is started after the measuredtime.

[0311]FIG. 70 shows an example of such a technique. Upon detecting anoperation that requires the high voltage, a controller 871 provides thehigh voltage control terminal with a signal to activate the DC-DCconverter 836, and temporarily stops working. The activation signal isalso provided to a delay circuit 872, which provides the controller 871with a delayed signal after a predetermined time. In response to thesignal, the controller 871 restarts.

[0312] The control operation of FIG. 68 may be realized by amicrocomputer. It is not practical, however, to install themicrocomputer in the flash memory 835. The embodiment mentioned above,therefore, employs the delay circuit and logic circuits to achieve thecontrol operation.

[0313] Although the DC-DC converter 836 is disposed outside the flashmemory 835 in the embodiment, it may be incorporated in the flash memorywith the same control mechanism explained above. The DC-DC converter 836will be explained next.

[0314]FIGS. 71 and 72 show examples of DC-DC converters. The DC-DCconverter 881 of FIG. 71 employs an inductance element 886 and includesan oscillator 882 and a switch 883 for controlling a step-up operation.A switch 884 is turned ON and OFF in response to a signal from theoscillator 882. The inductance element 886 is a coil connected between adiode 885 and the normal power source Vcc. When the switch 884 is turnedON and OFF, the amplitude of a voltage at an input end of the diode 885increases according to the principle of a transformer. An output end ofthe diode 885 provides only high voltage components, thereby providing ahigh voltage output. Switching the switch 883 controls the turning ONand OFF of the switch 884, to control an increase in voltage. Componentsexcept the coil 886 are relatively compact, so that they may beincorporated in the flash memory 835. The coil 886 is difficult toreduce in size without deteriorating performance. When incorporating theDC-DC converter 836 in the flash memory 835, it is preferable,therefore, to arrange the coil 886 outside the flash memory 835.

[0315] The DC-DC converter 891 of FIG. 72 employs a capacitor 894 as apassive component. An increase in voltage is controlled by controllingan oscillator 892. It is also difficult to install the capacitor 894 inthe flash memory 835. When incorporating the DC-DC converter 891 in theflash memory 835, the capacitor 894 may be arranged outside the flashmemory 835.

[0316]FIG. 73 shows a package in which a flash memory 812 and a DC-DCconverter 813 are mounted on an element to which a passive component 814such as an inductance element or a capacitance element is attached.

[0317] A semiconductor memory device according to the ninth aspect ofthe present invention will be explained next with reference to FIGS. 74to 91.

[0318] Generally, semiconductor memory devices (for example, EEPROMs,flash memories) must be tested in a production line, and this testing isparticularly important for new products to analyze and guarantee them.To easily carry out the test, the semiconductor memory devices usuallyincorporate test functions such as full selection and nonselectionfunctions of word or bit lines. These functions are provided for adecoder circuit in the semiconductor memory devices, and are used whenthe test is carried out in the production line.

[0319] FIGS. 74 to 77 each show an example of a decoder arranged in aconventional decoder circuit of a semiconductor memory device. In FIG.74, the decoder has a NOR output circuit composed of CMOS transistors.The decoder includes p-channel type MOS transistors TRP1, TRP2, andTRP3, and n-channel type MOS transistors TRN3, TRN4, and TRN6. A sourceof the transistor TRP1 is connected to a high voltage supply source Vcc,and a source of the transistor TRN6 is connected to a low voltage supplysource Vss.

[0320] As shown in FIG. 74, a gate of the transistor TRP1 is connectedto a gate of the transistor TRN3, and an output of an address buffer AD1is supplied to these gates of the transistors TRP1 and TRN3. A source ofthe transistor TRP5 is connected to the high voltage supply source Vcc,and a drain of thereof is connected to an output end VOT of the decoder.A full selection control signal AH is supplied to gates of thetransistors TRP5 and TRN6, and a nonselection signal /An is supplied togates of the transistors TRP2 and TRN4. Note, a mark “/” denotes aninverted signal, and thus the signal /An denotes an inverted signal ofthe signal An. Further, the signal /An may be a specific terminal forreceiving a specific high signal (extremely high voltage signal), or thesignal /An may be a terminal for receiving a specific address signal.

[0321] This decoder of FIG. 74 usually achieves a decoder function onlywith the NOR circuit, and to achieve a test function, the decoder needsthe transistors TRP5 and TRN6 as control transistors in addition to thebasic decoder circuit. These transistors are set to high or lowpotential in response to a control signal, to carry out a full selectionoperation or a nonselection operation.

[0322] Note, in FIG. 74, the full selection control signal AH controlsthe gates of the transistors TRP5 and TRN6, to carry out the fullselection or nonselection operation. To carry out the full selectionoperation, the full selection control signal AH is set to low level L toturn ON the transistor TRP5, thereby maintaining a high potential on aword or bit line. On the other hand, to carry out the nonselectionoperation, the full selection and nonselection control signals AH and/An are each set to high level H to turn ON the transistors TRN4 andTRN6, thereby dropping the potential of the word or bit line.

[0323] This conventional arrangement increases the size and cost of thedecoder circuit. When semiconductor memory devices are more integrated,decoder layout pitches must be narrower making it difficult to achievethe test function.

[0324] The decoder of FIG. 75 resembles that of FIG. 74. In FIG. 74, adecoding section is a NOR circuit composed of CMOS transistors. FIG. 75differs from FIG. 74 in that FIG. 75 employs a full selection signal Aninstead of the total nonselection signal /An, and an inverter INV1 isconnected to the output end of the decoder. Namely, the logic of FIG. 75is opposite to that of FIG. 74.

[0325] The decoder of FIG. 76 differs from the decoder of FIG. 74 inthat it has an output section composed of a NAND circuit of CMOStransistors. The decoder of FIG. 76 includes p-channel type MOStransistors TRP11, TRP12, and TRP13, and n-channel type MOS transistorsTRN14, TRN15, and TRN16. A source of the transistor TRP11 is connectedto a high voltage supply source Vcc, and a source of the transistorTRN16 is connected to a low voltage supply source Vss. A source of thetransistor TRN15 is connected to the low voltage supply source Vss, anda drain thereof is connected to an output end of the decoder. A gate ofthe transistor TRP12 is connected to a gate of the transistor TRN14, anda gate of the transistor TRN14 is connected to an output of an addressbuffer AD2. A separate nonselection control signal AL is supplied togates of the transistors TRP11 and TRN15, and a full selection signal Anis supplied to gates of the transistors TRP13 and TRN16.

[0326] When the nonselection control signal AL is set to high level H,the transistor TRN15 is turned ON to drop the output of the decoder,thereby putting a word or bit line in an unselected state. When thenonselection and full selection control signals AL and An are each setto low level L, the transistors TRP11 and TRP13 are turned ON. As aresult, the decoder provides the high voltage Vcc to carry out fullselection.

[0327] The decoder of FIG. 77 resembles that of FIG. 76. A decodingsection of FIG. 77 is a NAND circuit of CMOS transistors. FIG. 77differs from FIG. 76 in that it employs a nonselection signal /Aninstead of the full selection signal An, and an inverter INV2 isconnected to an output end of the decoder. Namely, the logic of FIG. 77is opposite to that of FIG. 76. All of these conventional decoders hasthe problems mentioned before.

[0328] An object of the ninth aspect of the semiconductor memory deviceof the present invention is to provide an inexpensive decoder circuitthat solves the problems of the prior art. The decoder circuit of thepresent invention is simple and compact to achieve the full selectionand nonselection of word or bit lines in a test mode. This decodercircuit is suitable for high integration.

[0329] FIGS. 78 to 81B show embodiments of a decoder circuit of asemiconductor memory device according to the ninth aspect of the presentinvention.

[0330] As shown in FIG. 78, an output row or a decoding row of thedecoder circuit 901 is connected to a first power source 904 forsupplying a high voltage Vcc and to a second power source 905 forsupplying a low voltage (ground voltage) Vss or the high voltage Vcc inresponse to a control signal AH. The output row of the decoder circuit901 is composed of decoders DEC1 to DECn. Namely, each of the decodersDEC1 to DECn is connected to the first power source 904 for supplyingthe high voltage Vcc, and each of the decoders DEC1 to DECn receivesaddress signals A0 to Am and /A0 to /Am. The decoders DEC1 to DECn (902)have output ends VOT1 to VOTn, respectively.

[0331] In response to the control signal AH, the second power source 905supplies the low voltage Vss or the high voltage Vcc. The second powersource 905 is shared by the decoders DEC1 to DECn. Namely, an output endof the second power source 905 is connected to input ends of thedecoders DEC1 to DECn. A nonselection signal /An is supplied to an inputend of each of the decoders DEC1 to DECn.

[0332] Note, in the ninth aspect of the present invention, the secondpower source 905 may have any circuit arrangement if it can provide thefunction mentioned above. For example, the second power source 905 mayhave an inverter 906 as shown in FIG. 78. In response to the level ofthe control signal AH, the inverter 906 provides the high voltage Vcc orthe low voltage Vss.

[0333] The operation of the decoder circuit of FIG. 78 will be explainednext. To carry out a nonselection operation, the nonselection signal /Anand control signal AH are each set to high level H. The inverter 906provides the voltage Vss of low level L, and the decoders DEC1 to DECnconnected to the inverter 906 operate like conventional decoders. Theoutput ends VOT1 to VOTn of the decoders DEC1 to DECn are each at lowlevel L to establish a nonselection state. On the other hand, to carryout a full selection operation, the nonselection signal /An is set tohigh level H, and the control signal AH to low level L. The inverter 906provides the voltage Vcc of high level H, and the decoders DEC1 to DECnconnected to the inverter 906 receive the voltage Vcc, and the outputends VOT1 to VOTn of the decoders DEC1 to DECn each provide the voltageVcc of high level H to establish a full selection state.

[0334] In this way, the single second power source 905 is shared by thedecoders DEC1 to DECn, and therefore, no additional circuits nortransistors are required for the decoder circuit. This results insuppressing the size of the decoder circuit and easily carrying out thefull selection or nonselection operation of word or bit lines in a testmode.

[0335] The decoder circuit of FIG. 79 is basically the same as that ofFIG. 78 and is characterized by a decoding row 903 (dec1 to decn). Eachof the decoders is connected to a first power source 904 for supplying ahigh voltage Vcc, and receives decoder selection signals A0 to Am and/A0 to /Am. Output ends N1 to Nn of the decoders dec1 to decn areconnected to output terminals VOT1 to VOTn through inverters INV7-1 toINV7-n, respectively.

[0336] The decoders dec1 to decn share a second power source 905, whichsupplies a low (ground) voltage Vss or the high voltage Vcc in responseto a control signal AL. Namely, an output end of the second power source905 is connected to an input of each of the decoders dec1 to decn. Atotal selection signal An is supplied to an input of each of thedecoders dec1 to decn. The second power source 905 has an inverter 906.In response to the level of the control signal AL, the inverter 906provides the high voltage Vcc or the low voltage Vss.

[0337] The operation of FIG. 79 will be explained next. To carry out afull selection operation, the full selection signal An is set to highlevel H, and the control signal AL to high level H. The inverter 906provides the voltage Vss of low level L, and the decoders dec1 to decnconnected to the inverter 906 operate like the conventional decoders.The output ends N1 to Nn of the decoders dec1 to decn each provide thevoltage Vss of low level L. The low level signals are inverted by theinverters INV7-1 to INV7-n, so that the output ends VOT1 to VOTn eachprovide high level H, to establish a full selection state. On the otherhand, to achieve a nonselection operation, the full selection signal Anis set to high level H, and the control signal AL is set to low level L.The inverter 906 provides the voltage Vcc of high level H. Accordingly,the decoders dec1 to decn connected to the inverter 6 receive thevoltage Vcc, and the output ends N1 to Nn of the decoders dec1 to decneach provide the voltage Vcc of high level H. These high level outputsare inverted by the inverters INV7-1 to INV7-n, so that the output endsVOT1 to VOTn each become low level L to establish a nonselection state.

[0338] The decoder circuit of FIG. 80A is basically the same as that ofFIG. 78 but inverts the polarities of signals.

[0339] The decoder circuit 901 of FIG. 80A has an output row of decodersDEC1 to DECn. Each of the decoders DEC1 to DECn is connected to a firstpower source 904 for supplying a low voltage Vss, and receives addresssignals A0 to Am and /A0 to /Am. The decoders have output ends VOT1 toVOTn, respectively. In response to a control signal AL, the second powersource 905 supplies the low voltage Vss or the high voltage Vcc. Thesecond power source 905 is shared by the decoders DEC1 to DECn. Namely,an output end of the second power source 905 is connected to an inputend of each of the decoders DEC1 to DECn. A full selection signal An issupplied to each of the decoders DEC1 to DECn. The second power source905 has an inverter 906, and the inverter 906 provides the high voltageVcc or the low voltage Vss in response to the level of the controlsignal AL.

[0340] The operation of the embodiment of FIG. 80A will be explainednext. To carry out a full selection operation, the full selection signalAn and control signal AL are each set to low level L. The inverter 906provides the voltage Vcc of high level H, and the decoders DEC1 to DECnconnected to the inverter 906 operate like conventional decoders. Theoutput ends VOT1 to VOTn of the decoders DEC1 to DECn each become highlevel H to establish a full selection state. On the other hand, to carryout a nonselection operation, the full selection signal An is set to lowlevel L, and the control signal AL is set to high level H. The inverter906 provides the voltage Vss of low level L. Accordingly, the decodersDEC1 to DECn connected to the inverter 906 receive the voltage Vss, andthe output ends VOT1 to VOTn each become low level L to establish anonselection state.

[0341]FIG. 80B shows a modification of the decoder circuit shown in FIG.80A.

[0342] As shown in FIG. 80B, in this modification, two second powersources 905 a and 905 b are provided for the decoder circuit 901 havingan output row of decoders DEC1 to DECn. Namely, the power source 905 ais provided for the decoders DEC1 to DECk, and the power source 905 b isprovided for the decoders DECk+1 to DECn. For example, the total numberof the decoders DEC1 to DECn is specified as 1026 or 512, and each ofthe power sources 905 a and 905 b is provided for 512 or 256 decodersDEC1 to DECk and DECk+1 to DECn, respectively. Note, the control signalAL applied to inverters 906 a and 906 b of the power sources 905 a and905 b is the same signal. Further, the number of the power sources isnot limited to two, but can be specified as four or more. In this case,the decoders DEC1 to DECn are divided into four or more plural groups,and each of the power sources is provided for each of the dividedgroups, respectively. In this modification shown in FIG. 80B,transistors constituting the inverters 906 a and 906 b can be formed ina smaller size than that of the inverter 906 shown in FIG. 80A.

[0343] The decoder circuit of FIG. 81A is basically the same as that ofFIG. 80A and is characterized by a decoding row 903.

[0344] The decoding row 903 includes decoders dec1 to decn. Each ofthese decoders is connected to a first power source 904 for supplying alow voltage Vss, and receive decoder selection signals A0 to Am and /A0to /Am. Output ends N1 to Nn of the decoders dec1 to decn are connectedto output terminals VOT1 to VOTn through inverters INV7-1 to INV7-n,respectively.

[0345] The decoders dec1 to decn share a second power source 905, whichsupplies the low voltage Vss or a high voltage Vcc in response to acontrol signal AH. Namely, an output end of the second power source 905is connected to an input end of each of the decoders dec1 to decn. Anonselection signal /An is supplied to each of the decoders dec1 todecn. The second power source 905 has an inverter 906. In response tothe control signal AH, the inverter 906 provides the high voltage Vcc orthe low voltage Vss.

[0346] The operation of the embodiment of FIG. 81A will be explained. Toestablish a nonselection state, the nonselection signal /An and controlsignal AH are each set to low level L.

[0347] The inverter 906 provides the voltage Vcc of high level H, andthe decoders dec1 to decn connected to the inverter 906 operate likeconventional decoders. The output ends N1 to Nn of the decoders eachprovide the voltage Vcc of high level H. These high-level outputs areinverted by the inverters INV7-1 to INV7-n, and the output terminalsVOT1 to VOTn each provide low level L to establish the nonselectionstate. On the other hand, to establish a full selection state, thenonselection signal /An is set to low level L and the control signal AHto high level H. The inverter 906 provides the voltage Vss of low levelL. Accordingly, the decoders dec1 to decn connected to the inverter 906each receive the voltage Vss, and the output ends N1 to Nn of thedecoders each provide the voltage Vss of low level L. These low-leveloutputs are inverted by the inverters INV7-1 to INV7-n, and the outputterminals VOT1 to VOTn each provide high level H to establish the fullselection state.

[0348] In this way, the decoders DEC1 to DECn share the single secondpower source 905. Namely, the decoder circuit according to the presentinvention requires no additional circuits or transistors. This resultsin realization of a compact decoder circuit that easily carries out afull selection or nonselection operation of word or bit lines.

[0349]FIG. 81 shows a modification of the decoder circuit shown in FIG.81A.

[0350] As shown in FIG. 81B, in this modification, two second powersources 905 a and 905 b are provided for the decoder circuit 901 havingan output row of decoders dec1 to decn. Namely, the power source 905 ais provided for the decoders dec1 to deck, and the power source 905 b isprovided for the decoders deck+1 to decn. For example, the total numberof the decoders dec1 to decn is specified as 1026 or 512, and each ofthe power sources 905 a and 905 b is provided for 512 or 256 decodersdec1 to deck and deck+1 to decn, respectively. Note, similar to themodification of FIG. 80B, the control signal AH applied to inverters 906a and 906 b of the power sources 905 a and 905 b is the same signal.Further, the number of the power sources is not limited to two, but canbe specified as four or more plural. In this case, the decoders DEC1 toDECn are divided into four or more plural groups, and each of the powersources is provided for each of the divided groups, respectively. Inthis modification shown in FIG. 81B, transistors constituting theinverter 906 a and 906 b can be formed in a smaller size than that ofthe inverter 906 shown in FIG. 81A.

[0351]FIG. 82 shows an example of a semiconductor memory device (flashmemory) using a decoder circuit according to the ninth aspect of thepresent invention. In FIG. 82, reference 912 denotes a row decoder, 914denotes a column decoder, 917 denotes a sense amplifier, and 919 denotesa source voltage supply circuit. Note, in FIG. 82, the row decoder 912includes a row address buffer, and the column decoder 914 includes acolumn address buffer.

[0352] As shown in FIG. 82, the semiconductor memory device comprisesthe row decoder 912, the column decoder 914, a sense amplifier 917, andthe source voltage supply circuit 919. Further, the semiconductor memorydevice comprises memory cells (cell transistor) MCs each made of ann-channel type MOS (MIS) transistor, word lines WLs, bit lines BLs, andsource lines SLs. The source voltage supply circuit 519 is connected tothe sources of the memory cells MCs in a memory cell array through thesource lines SLs, to electrically collectively erase the memory cells.The decoder circuit according to the ninth aspect of the presentinvention is applied to the row decoder 912 or column decoder 914. Note,the decoder circuit of the ninth aspect of the present invention notonly has a normal decoding function, but also has a test function tocarry out a full selection or nonselection operation of word or bitlines.

[0353]FIGS. 83A and 83B show the details of the decoder circuit of FIG.78, and FIG. 83B shows transistors forming the decoder circuit.

[0354] In FIG. 83A, the decoder circuit 901 has the output row 902involving the decoders DEC1 to DECn. Each of the decoders DEC1 to DECnhas a NAND gate 920 for receiving the address signals A0 to Am and /A0to /Am, and a NOR gate 921 for receiving an output of the NAND gate 920and the nonselection signal /An. An output of the NOR gate 921 isconnected to a corresponding one of the output terminals VOT1 to VOTn.Other arrangements and operation of FIG. 83A are the same as those ofFIG. 78.

[0355]FIG. 83B shows transistors forming the elements of FIG. 83A. TheNAND gate 920 has p-channel type MOS transistors TRP91 and TRP92, andn-channel type MOS transistors TRN93 l to TRN93 n. These transistors aredisposed between the high voltage supply source Vcc and the low voltagesupply source Vss. The NOR gate 921 may be a known NOR gate such as theone shown in FIG. 74. The inverter 906 may be a known CMOS inverter(transistors TRP906 and TRN906).

[0356] The operation of the decoder circuit of FIGS. 83A and 83B is thesame as that of the decoder circuit of FIG. 78, and therefore, will notbe explained again.

[0357]FIGS. 84A and 84B show the details of the decoder circuit of FIG.79, in which FIG. 84B shows transistors forming the decoder circuit.

[0358] In FIG. 84A, the decoder circuit 901 has the decoder row 903involving the decoders dec1 to decn. The decoder circuit also has theinverters INV7-1 to INV7-n that are not provided for the decoder circuitof FIG. 83A. The operation of the decoder circuit of FIGS. 84A and 84Bis the same as that of the decoder circuit of FIG. 79, and therefore,will not be explained again.

[0359]FIGS. 85A and 85B show the gates and transistors of the decodercircuit of FIG. 80A.

[0360] The decoder circuit 901 of FIGS. 85A and 85B differ from those ofFIGS. 83A, 83B and 84A, 84B in that each decoder of the decoder circuit901 is formed of NAND gates 920 and 920′. Note, the NAND gate 920 is thesame as that of FIGS. 83A and 83B, and the NAND gate 920′ is the same asthat of FIGS. 76 and 77. The inverter 906 is a known one. The operationof the decoder circuit of FIGS. 85A and 853 is the same as that of thedecoder circuit of FIG. 80A, and therefore, will not be explained again.Note, the transistors TRP906 and TRN906 constituting the inverter 906may be formed of larger size transistors than the other transistors (forexample, transistors TRP91, TRP91 and TRN93, TRN9 n) to adequately passtransient currents caused by parasitic capacitances, and the like.Further, as described with reference to FIGS. 80A and 80B, when thedecoders (DEC1 to DECn) are divided into a plurality of groups (DEC1 toDECk, DECk+1 to DECn) and a plurality of inverters (906 a and 906 b) areprovided for the divided groups, the transistors (TRP906, TRN906)constituting each of the inverters can be formed in a smaller size thanthat of the inverter 906 provided for all of the decoders (DEC1 toDECn).

[0361]FIGS. 86A and 86B show the gates and transistors of the decodercircuit of FIG. 81A.

[0362] The decoder circuit of FIGS. 86A and 86B resembles that of FIGS.85A and 85B and differs from those of FIGS. 83A, 835 and 84A, 84B inthat each decoder of the decoder circuit is formed of NAND gates 920 and920′. The NAND gate 920 is identical to that of FIGS. 83A and 83B, andthe NAND gate 920′ is identical to that of FIGS. 76 and 77. The inverter906 is a known one. The operation of the decoder circuit of FIGS. 86Aand 86B is the same as that of the decoder circuit of FIG. 81A, andtherefore, will not be explained again. Note, the transistors TRP906 andTRN906 constituting the inverter 906 may be formed as larger sizetransistors than the other transistors to adequately pass transientcurrents caused by parasitic capacitances, and the like. Further, asdescribed with reference to FIGS. 81A and 81B, when the decoders (dec1to decn) are divided into a plurality of groups (dec1 to deck, deck+1 todecn) and a plurality of inverters (906 a and 906 b) are provided forthe divided groups, the transistors (TRP906, TRN906) constituting eachof the inverters can be formed of a smaller size than that of theinverter 906 provided for all of the decoders (dec1 to decn).

[0363] Any one of the decoder circuits according to the ninth aspect ofthe present invention achieves a word line full selection function, aword line nonselection function, a bit line full selection function, anda bit line nonselection function. The ninth aspect of the presentinvention realizes these functions only by providing a second powersource for the decoder circuit involving a plurality of decoders. Thesecond power source has a switch including an inverter for selecting oneof at least two power source voltages. The ninth aspect of the presentinvention realizes these functions without increasing the size of thedecoder circuit. This decoder circuit, therefore, is appropriate for anintegrated semiconductor memory device.

[0364]FIG. 87 shows a power supply circuit for the decoder used in asemiconductor memory device according to the ninth aspect of the presentinvention. In FIG. 87, a reference 905′ denotes a second power sourcecorresponding to the inverter 906 shown in, for example, FIGS. 85A and85B, a reference EE denotes an erase control signal, and furtherreferences 920 and 920′ denote NAND gates which are the same as those ofFIGS. 85A and 85B. Note, an output of the power source 905′ is changedto a specific negative voltage (for example, −10 volts), when an eraseoperation is carried out.

[0365] As shown in FIG. 87, the power source 905′ comprises p-channeltype MOS transistors TRP951, TRP952, TRP953, TRP954, TRP955, and TRP956,and n-channel type MOS transistors TRN951, TRN952, TRN953, TRN954,TRN955, and TRN956. Note, the erase control signal EE is at a high levelH when read or write operations are carried out, and the erase controlsignal EE is at a low level L when an erase operation is carried out.Further, the transistor (transfer gate) TRP954 is used to cut a negativevoltage, and the transistors TRN954, TRN955, and TRN956 are formed on atriple well configuration. Further, source voltages V_(BB) of thetransistors TRN954, TRN955, and TRN956 can be specified as a normal lowvoltage (ground voltage) Vss in the read or write operations. Inaddition, the output voltage of the power source 905′ is applied to theother decoders. In this embodiment, an address signal ADD indicating aspecific sector is applied to the NAND gates (predecoder) 920, theoutput voltage of the power source 905′ is applied to the decodersincluded in the specific sector.

[0366] As described above, the decoder circuit of the ninth aspect ofthe present invention can be applied to a flash memory for carrying outan erase operation (electrical and collective erase operation) thereof.

[0367] In the above embodiments, the decoder circuit achieves a wordline full selection function, a word line nonselection function, a bitline full selection function, and a bit line nonselection function, in atest mode, but the decoder circuit of the ninth aspect of the presentinvention can also be applied to a flash memory having a collectiveerasing operation. Namely, in the erasing operation of the flash memory,all of the word lines must be collectively changed to a specificnegative voltage, e.g., −10 volts, and the circuit configuration of FIG.87 realizes this erase operation of the flash memory.

[0368] Recently, high function semiconductor devices (semiconductormemory devices) such as flash memories have been driven with lowvoltage, and some semiconductor memory devices have even driven with anegative voltage. When driving semiconductor memory devices ofconventional specifications with such low or negative voltage, thedecoder circuit according to the ninth aspect of the present inventionis advantageous for switching voltages from one to another.

[0369] When a semiconductor memory device carries out various tests, anormal voltage must be switched to a higher voltage than the normalvoltage to set a test mode. If such a high voltage is applied to thesemiconductor device by mistake, the test mode will unexpectedly startto cause malfunctions and destroy data. Therefore, the high voltage(extremely high voltage: for example, 12 volts) for starting the testmode in a semiconductor memory device is usually extremely high comparedwith a voltage (normal high voltage: for example, 5 volts) for normaloperation.

[0370]FIG. 88 shows a detector 930 for detecting the extremely highvoltage for starting the test mode.

[0371] A p-channel type MOS transistor TRP23 and an n-channel type MOStransistor TRN23 are connected to each other in series. A node N23between the transistors TRP23 and TRN23 is connected to a buffer BF23,which is connected to an output end TEST. The output end TEST isconnected to a test circuit, and gates of the transistors TRP23 andTRN23 are connected to a high voltage supply source Vcc. A source of thetransistor TRP23 is connected to a power source VIN, which provides anextremely high voltage much higher than the high voltage Vcc. A sourceof the transistor TRN23 is connected to a low voltage supply source Vssor to ground. The high voltage detector 930 causes no problems if thehigh voltage supply source Vcc is stable. When the high voltage supplysource Vcc is unstable, for example, at activation, the problemmentioned before occurs.

[0372] Referring to FIG. 89, the high voltage Vcc is applied to thesemiconductor device (semiconductor memory device) at time t₀ and isstabilized after time t₁. A pulse P of the extremely high voltage VINhigher than the voltage Vcc is applied at time t₁. Then, a sourcevoltage of the transistor TRP23 becomes higher than a gate voltagethereof. If the difference between the source and gate voltages exceedsa threshold voltage of the transistor TRP23, the transistor is turnedON, and the terminal TEST provides a test signal T to start apredetermined test mode.

[0373] The high voltage Vcc is unstable during a period between t₀ andt₁. If the voltage VIN rises quickly during this period and if apotential difference between the voltages VIN and Vcc exceeds, at timet₁, the threshold voltage of the transistor TRP23, the terminal TESTwill provide a test signal TE to unexpectedly start the test mode. Thisresults in a malfunction.

[0374] This problem is usually caused when the voltage VIN is appliedbefore the application of the high voltage Vcc.

[0375]FIG. 90 shows a test mode signal detector of a semiconductordevice (semiconductor memory device) according to the ninth aspect ofthe present invention.

[0376] The detector 930 has a first detector section K1, a seconddetector section K2, and an operation circuit E1. The first detectorsection K1 has a p-channel type MOS T81 and an n-channel type MOStransistor T82, and these transistors T81 and T82 are connected to eachother in series. A node between the transistors T81 and T82 forms afirst output end N81 of the first detector section K1. Gates of thetransistors T81 and T82 are connected to a high voltage supply sourceVcc, and a source of the transistor T81 is connected to an externalinput terminal VIN. A source of the transistor T82 is connected to a lowvoltage supply source Vss or to a ground GND.

[0377] The second detector section K2 has a p-channel type MOStransistor T83 and an n-channel type MOS transistor T84, and thetransistors T83 and T84 are connected to each other in series. A nodebetween the transistors T83 and T84 forms a second output end N82 of thesecond detector section K2. Gates of the transistors T83 and T84 areconnected to the high voltage supply source Vcc. A source of thetransistor T83 is connected to the external input terminal VIN, and asource of the transistor T84 is connected to the low voltage supplysource Vss or to the ground GND. The operation circuit E1 carries out alogical operation according to output signals from the detector sectionsK1 and K2, to provide a test signal TEST.

[0378] This detector effectively prevents application of an unexpectedtest signal, with no regard to the order of application of voltages. Thedetector detects the extremely high voltage VIN only on an expectedoccasion to start a test mode.

[0379] In FIG. 90, the supply source of the high voltage Vcc may be aninternal power source or an external power source. The external inputterminal VIN provides the test mode starting voltage (extremely highvoltage) that is higher than the high voltage Vcc. The operation circuitE1 may be a NAND gate. An output end of the circuit E1 may have aninverter INV, if necessary.

[0380] During a normal state with the high voltage Vcc being stable, thetransistor T83 is ON, and the transistor T84 is OFF. Accordingly, theoutput end N82 of the detector section K2 provides a signal of highlevel H, and therefore, the test terminal TEST provides an output signalof the output terminal N81 of the detector section K1, thereby securingthe same operation as the conventional circuit.

[0381] When the high voltage Vcc is applied, there is a risk that theoutput end N81 of the detector section K1 may provide a signal of highlevel H by mistake. When the extremely high voltage VIN is higher thanthe voltage Vcc and a voltage difference between them exceeds thethreshold voltage of the transistor T81, the transistor T83 is weaklyturned ON or is turned OFF to turn ON the transistor T84. As a result,the output end N82 of the second detector section K2 provides a signalof low level L, and therefore, the test terminal TEST is maintained atlow level L.

[0382] Accordingly, as shown in FIG. 91, the embodiment of the ninthaspect of the present invention completely prevents the unexpected startof the test mode.

[0383]FIG. 92 shows a decoder circuit shown in FIG. 85B, using thedetectors 930 shown in FIG. 90.

[0384] As shown in FIG. 92, two detectors 930 a and 930 b are providedfor applying signals An and AL. Namely, the detector 930 a receives anaddress signal An through a terminal (An), and when a voltage level ofthe terminal An is at an extremely high voltage (for example, 12 volts)higher than a normal high voltage (for example, 5 volts) for starting atest mode in a semiconductor memory device, a test signal TEST of a highlevel H is output therefrom. Note, when the test signal TEST is at ahigh level H, the decoder circuit 902 (901) carried out a test mode offull selection or nonselection functions of word or bit lines. Further,the detector 930 b receives a signal TT through a terminal (TT), andwhen a voltage level of the terminal TT is at an extremely high voltagehigher than a normal high voltage, the decoder circuit 901 (decoder 902)is set to a full selection mode. On the other hand, when a voltage levelof the terminal TT is at a low voltage, the decoder circuit 901 (decoder902) is set to a nonselection mode.

[0385] Namely, when the test signal TEST is at a high level H, a signalAn is at a low level L. In this state, when a control signal AL is at alow level L, a full selection mode is set in a semiconductor memorydevice (for example, EEPROM, flash memory) by the decoder circuit 901.On the other hand, when the control signal AL is at a high level H, anonselection mode is set in the semiconductor memory device by thedecoder circuit 901. Note, in the normal operation (normal mode) of thesemiconductor memory device, the test signal TEST is at a low level L,that is the voltage level of the terminal An is at a normal voltage (forexample, 0 volts to 5 volts).

[0386] As described above, the detectors can be provided for settingfull selection or non selection function (test mode) in thesemiconductor memory device.

[0387] As explained above, a semiconductor memory device according tothe first aspect of the present invention effectively employs redundantword lines and achieves stable write and verify operations, therebyimproving the yield and performance of semiconductor memories.

[0388] A semiconductor memory device according to the second aspect ofthe present invention replaces defective real cells with redundant cellswithout increasing the size of a semiconductor memory device, therebyimproving the yield of large capacity semiconductor memories andreducing the cost thereof.

[0389] A semiconductor memory device according to the third aspect ofthe present invention carries out a delivery test of a semiconductormemory device with “n” rewrite operations at the maximum, taking intoaccount deterioration due to an increase in the number of rewriteoperations, to thereby guarantee the maximum rewrite operations N (N>n)for a user.

[0390] A semiconductor memory device according to the fourth aspect ofthe present invention provides a semiconductor memory device in whichthe threshold voltage of a write voltage supply transistor has noinfluence on a write drain voltage, so that data is correctly writteneven with a low write voltage.

[0391] A semiconductor memory device according to the fifth aspect ofthe present invention correctly reads data even if there are overerasedcell transistors.

[0392] A semiconductor memory device according to the sixth aspect ofthe present invention saves overerased cell transistors and correctlyreads data.

[0393] A semiconductor memory device according to the seventh aspect ofthe present invention simultaneously erases blocks of memory cells andeasily verifies the erasure.

[0394] A semiconductor memory device according to the eighth aspect ofthe present invention achieves a word line full selection function, aword line nonselection function, a bit line full selection function, anda bit line nonselection function. The eighth aspect of the presentinvention achieves these functions only by providing a second powersource for the decoder circuit containing a plurality of decoders. Thesecond power source circuit has a switch including an inverter forselecting one of at least two power source voltages. The eighth aspectof the present invention realizes these functions without increasing thesize of the decoder circuit. This decoder circuit is appropriate for anintegrated semiconductor device.

[0395] A semiconductor memory device according to the nineth aspect ofthe present invention employs two power sources but is as easy to handleas if it had only a single power source. This memory requires no step-upcircuit for the power sources and works with low voltages.

[0396] Many different embodiments of the present invention may beconstructed without departing from the spirit and scope of the presentinvention, and it should be understood that the present invention is notlimited to the specific embodiments described in this specification,except as defined in the appended claims.

1. A semiconductor memory device comprising: 2^(n) word lines; aplurality of bit lines; a plurality of nonvolatile memory cells eachformed of a MIS transistor disposed at each intersection of said wordlines and said bit lines, and a threshold voltage of said MIS transistorbeing externally electrically controllable; a write circuit for writingdata to a memory cell located at an intersection of selected ones ofsaid word lines and said bit lines; a sense amplifier for reading dataout of said memory cells; a first means for simultaneously selecting ablock of 2^(m) (n>m) word lines among said 2^(n) word lines; and asecond means for not selecting a block of 2^(k) (m>k) word lines amongsaid 2^(m) word lines, said second means not selecting said block of2^(k) word lines and selecting a block of 2^(k) word lines preparedoutside said 2^(n) word lines when any one of said 2^(k) word linesamong said 2^(m) word lines is defective.
 2. A semiconductor memorydevice as claimed in claim 1, wherein said selected word lines receive anegative voltage, and said unselected word lines receive a zero orpositive voltage.
 3. A semiconductor memory device as claimed in claim1, wherein said block of 2^(n) word lines forms a real cell block, saidblock of 2^(m) word lines forms an erase block, and said block of 2^(k)word lines outside said block of 2^(n) word lines forms a redundant cellblock.
 4. A semiconductor memory device as claimed in claim 1, whereinsaid semiconductor memory device is constituted by a flash memory.
 5. Asemiconductor memory device comprising: 2^(n) word lines; a plurality ofbit lines; a plurality of nonvolatile memory cells each formed of a MIStransistor disposed at each intersection of said word lines and said bitlines, and a threshold voltage of said MIS transistor being externallyelectrically controllable; a write circuit for writing data to a memorycell located at an intersection of selected ones of said word lines andsaid bit lines; a sense amplifier for reading data out of said memorycells; a first means for simultaneously selecting a block of 2^(m) (n>m)word lines among said 2^(n) word lines; and a second means for notselecting a block of 2^(k) (m>k) word lines among said 2^(m) word lines,data being written to any memory cell transistor, which is contained insaid 2^(k) word lines and whose threshold voltage is lower than thepotential of an unselected word line, so that the threshold voltage ofsaid memory cell transistor exceeds the potential of said unselectedword line, and a block of 2^(k) word lines prepared outside said 2^(n)word lines being used as redundant word lines.
 6. A semiconductor memorydevice as claimed in claim 5, wherein said semiconductor memory deviceis constituted by a flash memory.
 7. A semiconductor memory devicecomprising: a plurality of word lines; a plurality of bit lines; aplurality of nonvolatile memory cells each formed of a MIS transistordisposed at each intersection of said word lines and said bit lines, anda threshold voltage of said MIS transistor being externally electricallycontrollable; a write circuit for writing data to a memory cell locatedat an intersection of selected ones of said word lines and said bitlines; and a sense amplifier for reading data out of said memory cells,wherein each word line is controlled such that a drain current of amemory cell transistor connected to said word line is lower than achannel current thereof, when writing data to said cell transistor toincrease the threshold voltage of said memory cell transistor to behigher than the potential of an unselected word line.
 8. A semiconductormemory device as claimed in claim 7, wherein each word line iscontrolled by applying a signal in accordance with a pulse signal.
 9. Asemiconductor memory device as claimed in claim 7, wherein saidsemiconductor memory device is constituted by a flash memory.
 10. Asemiconductor memory device comprising: a plurality of word lines; aplurality of bit lines; a plurality of nonvolatile memory cells eachformed of a MIS transistor disposed at each intersection of said wordlines and said bit lines, and a threshold voltage of said MIS transistorbeing externally electrically controllable; a write circuit for writingdata to a memory cell located at an intersection of selected ones ofsaid word lines and said bit lines; and a sense amplifier for readingdata out of said memory cells, wherein an output current of said senseamplifier is changed according to a combination of ON states of two loadtransistors having different capacities, to realize a normal data readoperation, an erase verify operation, and a write verify operation. 11.A semiconductor memory device as claimed in claim 10, wherein areference voltage is increased to provide a word line with a voltage,which is used to carry out said write verify or erase verify operationson any cell transistor connected to said word line.
 12. A semiconductormemory device as claimed in claim 10, wherein p-channel type andn-channel type transistors fabricated in the same process are connectedin series like diodes to provide a word line with a voltage which isused to carry out said write verify or erase verify operations on anycell transistor connected to said word line.
 13. A semiconductor memorydevice as claimed in claim 10, wherein said semiconductor memory deviceis constituted by a flash memory.
 14. A semiconductor memory devicecomprising: a plurality of real memory cells divided into blocks; aplurality of redundant memory cells to be replaced with defective onesof said real memory cells; a plurality of defective address specifyingmeans for specifying defective addresses of the respective blocks ofsaid real memory cells; and an address comparing means shared by saiddefective address specifying means, for comparing said defectiveaddresses with addresses in said blocks of said real memory cells.
 15. Asemiconductor memory device as claimed in claim 14, wherein saidsemiconductor memory device comprises: a redundant circuit having saiddefective address specifying means and said address comparing means; areal cell selecting means for receiving an output of said redundantcircuit and a block address for specifying one of said real cell blocks,and controlling the selection and non-selection of said memory cells insaid specified real cell block; and a redundant cell selecting means forreceiving the output of said redundant circuit and said block address,and controlling the selection and nonselection of said redundant cells.16. A semiconductor memory device comprising: a real cell array having aplurality of memory cells, and a plurality of redundant cells to bereplaced with defective memory cells of said real cell array; aredundant information storing cell array for writing data to a defectiveaddress according to an externally provided address; a cell selectioncircuit for selecting said redundant information storing cell arrayaccording to the externally provided address; and a read circuit forreading an output of said redundant information storing cell arrayselected by said cell selection circuit, and providing a redundancysignal.
 17. A semiconductor memory device as claimed in claim 16,wherein said redundant information storing cell array and said readcircuit are provided in plural, outputs of said read circuits areoperated, and thereby a plurality of redundancy signals are output. 18.A semiconductor memory device as claimed in claim 16, wherein saidredundant information storing cell array is constituted by a pluralityof nonvolatile memory cell transistors.
 19. A semiconductor memorydevice comprising electrically erasable nonvolatile memory cells to andfrom which data is automatically written and erased according to aninternal algorithm incorporated in said semiconductor memory device,wherein the allowable value of write or erase operations is carried outaccording to said internal algorithm being variable.
 20. A semiconductormemory device as claimed in claim 19, wherein said semiconductor memorydevice is constituted by a flash memory, and the allowable value ofwrite or erase operations is determined by the number of write pulses orerase pulses.
 21. A semiconductor memory device as claimed in claim 20,wherein the maximum number of said pulses to be applied in a deliverytest is reduced to impose severer conditions on said delivery test. 22.A semiconductor memory device as claimed in claim 19, wherein saidsemiconductor memory device is constituted by a flash memory, and theallowable value of write or erase operations is determined by the widthof write pulses or erase pulses.
 23. A semiconductor memory device asclaimed in claim 22, wherein the maximum width of said pulses to beapplied in a delivery test is reduced to impose severer conditions onsaid delivery test.
 24. A semiconductor memory device as claimed inclaim 19, wherein a change of the allowable value of write or eraseoperations is carried out by applying a high voltage to a specificterminal of said semiconductor memory device.
 25. A semiconductor memorydevice comprising: a plurality of word lines; a plurality of bit lines;a plurality of memory cells each formed of a MIS transistor disposed ateach intersection of said word lines and said bit lines, and a thresholdvoltage of said MIS transistor being externally electricallycontrollable; and a write voltage supply transistor for supplying awrite voltage to a drain of said memory cell, wherein said write voltagesupply transistor is formed of a p-channel type MIS transistor, whicheffectively applies the write voltage to the drain of said memory cell.26. A semiconductor memory device as claimed in claim 25, wherein saidsemiconductor memory device is constituted by a flash memory.
 27. Asemiconductor memory device comprising: a plurality of word lines; aplurality of bit lines; a plurality of memory cells each formed of a MIStransistor disposed at each intersection of said word lines and said bitlines, and a threshold voltage of said MIS transistor being externallyelectrically controllable; and a write voltage supply transistor forsupplying a write voltage to a drain of said memory cell, wherein saidwrite voltage supply transistor is formed of an n-channel type MIStransistor, and said semiconductor memory device comprises a step-upmeans being disposed to increase a gate voltage of said write voltagesupply transistor at least up to a sum of the write voltage and athreshold voltage of said write voltage supply transistor.
 28. Asemiconductor memory device as claimed in claim 27, wherein saidsemiconductor memory device is constituted by a flash memory.
 29. Asemiconductor memory device comprising: a plurality of word lines; aplurality of bit lines; a memory cell array including a plurality ofmemory cells each formed of a MIS transistor disposed at eachintersection of said word lines and said bit lines, a threshold voltageof said MIS transistor being externally electrically controllableaccording to charges to be injected to a floating gate thereof, and thefloating gates of said MIS transistors being simultaneously dischargedto collectively erase said memory cells; a first power source forapplying a normal selection voltage to a selected word line to selectmemory cells connected to said word line, when reading data; and asecond power source for establishing an unselected state on unselectedword lines including memory cells that have been overerased by saidcollective erasing, when reading data.
 30. A semiconductor memory deviceas claimed in claim 29, wherein said memory cells are formed ofenhancement n-channel type MIS transistors, said first power source is apositive voltage source for generating a normal positive voltage, andsaid second power source is a negative voltage source for generating apredetermined negative voltage for cutting OFF said overerased n-channeltype MIS transistors that function as depletion transistors due to saidovererasing.
 31. A semiconductor memory device as claimed in claim 29,wherein said semiconductor memory device is constituted by a flashmemory.
 32. A semiconductor memory device comprising: a plurality ofword lines; a plurality of bit lines; a memory cell array including aplurality of memory cells each formed of a MIS transistor disposed ateach intersection of said word lines and said bit lines, a thresholdvoltage of said MIS transistor being externally electricallycontrollable according to charges to be injected to a floating gatethereof, and the floating gates of said MIS transistors beingsimultaneously discharged to collectively erase said memory cells; afirst row decoder for applying a normal voltage to a selected word lineto select memory cells connected to said word line, when reading data;and a second row decoder for applying a predetermined source voltage tothe source of each memory cell connected to said selected word line, andapplying an unselected state establishing voltage to the sources ofmemory cells, including those overerased by said collective erasing,connected to unselected word lines, when reading data.
 33. Asemiconductor memory device as claimed in claim 32, wherein said memorycells are formed of enhancement n-channel type MIS transistors, and saidsecond row decoder applies a low source voltage to the source of eachcell transistor connected to said selected word line, and applies avoltage higher than the level of a selected bit line to the sources ofmemory cells connected to said unselected word lines.
 34. Asemiconductor memory device as claimed in claim 33, wherein said secondrow decoder applies a voltage equal to the level of said selected bitline to the sources of memory cells connected to said unselected wordlines, when reading data.
 35. A semiconductor memory device as claimedin claim 32, wherein said semiconductor memory device is constituted bya flash memory.
 36. A semiconductor memory device comprising: aplurality of word lines; a plurality of bit lines; a memory cell arrayincluding a plurality of memory cells each formed of a MIS transistordisposed at each intersection of said word lines and said bit lines, athreshold voltage of said MIS transistor being externally electricallycontrollable according to charges to be injected to a floating gatethereof, and the floating gates of said MIS transistors beingsimultaneously discharged to collectively erase said memory cells,wherein a method of saving overerased memory cells of said semiconductormemory device detects memory cells that have been overerased by saidcollective erasing, and writing data to said overerased memory cells,thereby saving said overerased memory cells.
 37. A semiconductor memorydevice comprising: a plurality of word lines; a plurality of bit lines;a memory cell array including a plurality of memory cells each formed ofa MIS transistor disposed at each intersection of said word lines andsaid bit lines, and a threshold voltage of said MIS transistor beingexternally electrically controllable according to charges to be injectedto a floating gate thereof; a write-before-erase means for writing allmemory cells of said memory cell array before erasing them; an erasemeans for erasing all of said written memory cells by saidwrite-before-erase means and for verifying said erasing; an overerasedcell detecting means for detecting overerased memory cells among saiderased and verified memory cells by said erase means; and an overerasedcell saving means for writing said overerased memory cells detected bysaid overerased cell detecting means, thereby saving said overerasedmemory cells.
 38. A semiconductor memory device comprising: a pluralityof word lines; a plurality of bit lines; and a plurality of nonvolatilememory cells each formed of a MIS transistor disposed at eachintersection of said word lines and said bit lines, and a thresholdvoltage of said MIS transistor being externally electricallycontrollable, wherein said nonvolatile memory cells are divided into aplurality of cell blocks to be selected according to a block selectionsignal provided by a block address buffer, each of said cell blocks hasa data erasing means and a latching means for latching said blockselection signal, and thereby data of said cell blocks that have latchedsaid block selection signal are simultaneously erased.
 39. Asemiconductor memory device as claimed in claim 38, wherein saidsemiconductor memory device comprises data decision circuits fordiscriminating cell data in said respective cell blocks, expected valuestorage circuits each for storing an expected value for write andwrite-verify operations as well as an expected value for an erase-verifyoperation, coincidence circuits each for comparing an output signal ofsaid data decision circuit with the expected value and providing acoincidence signal, and a logic circuit for providing a logical multiplyof the coincidence signals from said respective cell blocks.
 40. Asemiconductor memory device as claimed in claim 38, wherein saidsemiconductor memory device comprises data decision circuits fordiscriminating cell data in said respective cell blocks, expected valuegenerators each for generating an expected value for write andwrite-verify operations as well as an expected value for an erase-verifyoperation, coincidence circuits each for comparing an output signal ofsaid data decision circuit with the expected value and providing acoincidence signal, and a logic circuit for providing a logical multiplyof the coincidence signals from said respective cell blocks.
 41. Asemiconductor memory device as claimed in claim 38, wherein saidsemiconductor memory device comprises data decision circuits fordiscriminating cell data in said respective cell blocks, data inversioncircuits each for inverting an output signal of said data decisioncircuit in accordance with erase and write operations, and a logiccircuit for providing a logical multiply of said data inversion circuitsfrom said respective cell blocks.
 42. A semiconductor memory devicecomprising: a first terminal for receiving a normal voltage; a secondterminal for receiving a high voltage from a high-voltage supply means,and said high voltage being required to write or erase data and higherthan said normal voltage required to read data; and a third terminal forproviding said high-voltage supply means with a control signal thatcontrols the supply of said high voltage.
 43. A semiconductor memorydevice as claimed in claim 42, wherein said semiconductor memory devicecomprises a command determination means that determines whether or notan operation specified by an input command to said semiconductor memorydevice requires said high voltage, and provides a control signal tostart the supply of said high voltage if the operation requires saidhigh voltage, and if not, a control signal to stop said high voltage.44. A semiconductor memory device as claimed in claim 42, wherein saidsemiconductor memory device comprises a voltage test means that testswhether or not said supplied high voltage is greater than apredetermined value, said high voltage requiring operation being startedif said supplied high voltage is greater than said predetermined value.45. A semiconductor memory device as claimed in claim 42, wherein saidsemiconductor memory device comprises a delay means that delays thestart of said high voltage requiring operation by a predetermined timeafter said control signal to start the supply of said high voltage issent.
 46. A semiconductor memory device as claimed in claim 42, whereinsaid semiconductor memory device is determined as a flash memory.
 47. Asemiconductor memory device comprising: a step-up circuit for supplyinga high voltage that is required to write or erase data and higher than anormal voltage required to read data; and a command determination meansthat determines whether or not an operation specified by an inputcommand to said semiconductor memory device requires the high voltage,and provides a control signal to start the supply of said high voltageif the operation requires said high voltage, and if not, a controlsignal to stop said high voltage.
 48. A semiconductor memory device asclaimed in claim 47, wherein a passive component of said step-up circuitis arranged on an outside of said semiconductor memory device.
 49. Asemiconductor memory device as claimed in claim 48, wherein said passivecomponent is an inductance element.
 50. A semiconductor memory device asclaimed in claim 48, wherein said passive component is a capacitanceelement.
 51. A semiconductor memory device as claimed in claim 47,wherein said semiconductor memory device comprises a voltage test meansthat tests whether or not said supplied high voltage is greater than apredetermined value, said high voltage requiring operation being startedif said supplied high voltage is greater than said predetermined value.52. A semiconductor memory device as claimed in claim 47, wherein saidsemiconductor memory device comprises a delay means that delays thestart of said high voltage requiring operation by a predetermined timeafter said control signal to start the supply of said high voltage issent.
 53. A semiconductor memory device as claimed in claim 48, whereinsaid semiconductor memory device and said passive component are sealedin the same package.
 54. A semiconductor memory device as claimed inclaim 47, wherein said semiconductor memory device is determined as aflash memory.
 55. A computer system having a semiconductor memory deviceas a part of a storage means and a step-up circuit for generating a highvoltage required to write and erase data to and from said semiconductormemory device, wherein said computer system comprises: a control meansfor automatically generating a control signal to control said step-upcircuit, in response to an access operation to said semiconductor memorydevice.
 56. A semiconductor memory device comprising: a plurality ofword lines; a plurality of bit lines; a plurality of memory cellsdisposed at each intersection of said word lines and said bit lines, anda decoder circuit for selecting said memory cell according to an addresssignal in a normal decoding function and for carrying out a fullselection operation or a nonselection operation of said word lines orsaid bit lines in a test function; and an output row or a decoding rowconnected to a first power source and a second power source, said firstpower source supplying a high voltage, and said second power sourcesupplying a reference voltage or said high voltage in response to acontrol signal.
 57. A semiconductor memory device as claimed in claim56, wherein said semiconductor memory device comprises: a first detectorincluding a first transistor of first conduction type, a secondtransistor of second conduction type connected to said first transistorin series, a node between said first and second transistors, forming afirst output end of said first detector, a high voltage supply sourceconnected to gates of said first and second transistors, an externalinput terminal connected to an end of said first transistor, and a lowvoltage supply source connected to an end of said second transistor; asecond detector including a third transistor of second conduction type,a fourth transistor of first conduction type connected to said thirdtransistor in series, a node between said third and fourth transistors,forming a second output end of said second detector, said high voltagesupply source connected to gates of said third and fourth transistors,said external input terminal connected to an end of said thirdtransistor, and said low voltage supply source connected to an end ofsaid fourth transistor; and an operation circuit carrying out a logicaloperation according to output signals from said first and seconddetectors, to provide a test signal to said decoder circuit.
 58. Asemiconductor memory device as claimed in claim 56, wherein saidsemiconductor memory device is determined as a flash memory.
 59. Asemiconductor memory device comprising: a plurality of word lines; aplurality of bit lines; a plurality of memory cells disposed at eachintersection of said word lines and said bit lines; a decoder circuitfor selecting said memory cell according to an address signal in anormal decoding function and for carrying out a full selection operationor a nonselection operation of said word lines or said bit lines in atest function; and an output row or a decoding row connected to a firstpower source and a second power source, said first power sourcesupplying a reference voltage, and said second power source supplying areference voltage or said high voltage in response to a control signal.60. A semiconductor memory device as claimed in claim 59, wherein saidsemiconductor memory device comprises: a first detector including afirst transistor of first conduction type, a second transistor of secondconduction type connected to said first transistor in series, a nodebetween said first and second transistors, forming a first output end ofsaid first detector, a high voltage supply source connected to gates ofsaid first and second transistors, an external input terminal connectedto an end of said first transistor, and a low voltage supply sourceconnected to an end of said second transistor; a second detectorincluding a third transistor of second conduction type, a fourthtransistor of first conduction type connected to said third transistorin series, a node between said third and fourth transistors, forming asecond output end of said second detector, said high voltage supplysource connected to gates of said third and fourth transistors, saidexternal input terminal connected to an end of said third transistor,and said low voltage supply source connected to an end of said fourthtransistor; and an operation circuit carrying out a logical operationaccording to output signals from said first and second detectors, toprovide a test signal to said decoder circuit.
 61. A semiconductormemory device as claimed in claim 59, wherein said semiconductor memorydevice is determined as a flash memory.